The battle lines are already being drawn. As the two horse race between Boris Johnson and Jeremy Hunt to become the next Prime Minister enters the final straight, calls for the eventual winner to urgently fast track a credible and ambitious net zero emission strategy are intensifying by the day.

Today London Mayor Sadiq Khan became the latest high profile figure to call on the next Prime Minister to step up efforts to deliver on the UK's climate pledges, launching a strongly worded attack on frontrunner Boris Johnson in the process.

Speaking at a City Hall event to mark the launch of London Climate Action Week, the Mayor highlighted Johnson's previous public questioning of the scientific consensus on climate change and more recent support for the UK's net zero target. Khan argued that full blown 'denial' of climate risks had been forced to the margins of British public life, but he expressed concerns that a 'denial' of the need to act was being replaced by a culture of 'delay' whereby political leaders accept the need to respond to climate threats but then fail to deliver sufficiently ambitious policies.

"For years, climate change deniers have attempted to thwart climate action," Khan said. "From the oil companies that knew for decades about the link between their products and global warming, but kept it hidden and refused to act, to the industry of lobbyists, dodgy think-tanks and pseudo-scientists that sprang up to cast doubt on what is undeniably a consensus view among the real scientists and experts. Not to mention the politicians - including President Trump - who shamefully sought, and still seek to undermine, climate science for their own ends."

He added that his predecessor at City Hall had once sought to align himself with such views. "Take Boris Johnson, who only just a few years ago seemed to question the human links to climate change, describing concerns about warming weather and human-made climate change as a 'primitive fear' that is 'without foundation'," he said. In previous newspaper columns, Johnson has questioned the scientific on climate change while generally arguing the UK should still take steps to tackle climate risks.

Khan continued by warning the real obstacle to climate action in the UK came not from 'deniers' who "know they've already lost the argument and that they're on the wrong side of history", but from those who are signed up to delivering on the UK's climate goals and then oppose decarbonisation policies. "On the face of it, these individuals may sound more reasonable - but their agenda is equally as threatening and pernicious as the one peddled by the deniers," he said. "They say we must act, but then they refuse to put in place the plans, action or funding we desperately need - often citing the costs to business and consumers as their pretext when we know the costs of not acting are far greater and far graver. We need to hold these climate delayers to account and put pressure on the next Prime Minister to urgently show real leadership."

Khan also reiterated some of his specific policy asks, which largely echo long-standing calls from environmental campaign groups and green businesses. He called on the next Prime Minister to pull forward the target date to end the sale of fossil fuelled cars from 2040 to 2030, axe the planned VAT increase on some solar systems, increase support for community energy products, devolve more green building standards to local authorities, and formally declare a climate emergency and launch a 'Green New Deal'.

He stressed that bolder climate policies could benefit the economy, noting that in London total sales of Low Carbon Goods and Services have grown from £20.9bn to £39.7bn over the last decade, a total increase of 90 per cent, with a 20 per cent increase between 2015/16 and 2017/18 alone.

The Conservative Party leadership race has been characterized by broad support for the UK's new net zero emission target, with all the candidates agreeing more ambitious climate policies were urgently needed. Hunt and Johnson have both said they will prioritise action to deliver on the new goal, but in a race dominated by the UK's looming Brexit date neither candidate has provided much detail on how they plan to put the country back on track to meet its emissions goals.

Moreover, with Johnson still the runaway favourite to secure the keys to Number 10 concerns remain amongst green groups over how his net zero strategy will pan out. Speaking at a hustings event this weekend, he reiterated his commitment to climate action and said he was looking forward to taking up the issue with US President Donald Trump, arguing that he would be "very clear" that the US "must play a leading role in continuing to reduce CO2".

However, he also heaped praise on Trump's handling of the economy and emphasised his own free market instincts. "This is a guy who, when all is said and done, has got the US economy motoring along at about 3.6 per cent growth," he said. "He's put in capital allowances for business in a way that I think we should be looking at here, he's cut regulations and he's cut taxes in a way that has driven growth in the US… We Tories, we Conservatives, I think, for too long have failed to talk up the agenda of free market economics, and we've failed to be positive about it."

And upon hearing the arrival of his rival's helicopter at the event in Carlisle, he joked that any emissions could simply be offset. "Whose chopper is that?" he asked. "I've got a terrible feeling that it's mine. A deeply embarrassing moment, folks, just as I'm talking about climate change. We will make sure that we try and offset the global emissions by planting a shrubbery or whatever is necessary to do."

Johnson's historic support for deregulation combined with speculation about his likely cabinet appointments have fuelled concerns amongst green campaigners that his warm words on climate action could be undermined by a desire to rollback some environmental protections post-Brexit and resist any decarbonisation policies that involve state intervention.

Writing on Twitter this weekend, Amy Mount of the Greener UK group highlighted Times reports that Johnson was considering a "Trump-style" moratorium in the event of a 'no deal' Brexit, warning a freeze on new regulations could badly undermine efforts to tackle escalating climate risks.

She also warned that plans from both Johnson and Hunt for a 'war chest' to address the economic fallout from a 'no deal' Brexit were a damaging distraction from the need to come forward with more credible environmental plans.

A £20bn 'war chest' & Cobra-style task force - this is the sort of response needed to address real, urgent ENVIRONMENTAL CRISIS. It's an outrage that the leadership contest is being dominated by plans for a damaging, avoidable no-deal Brexit instead. https://t.co/cf2dhHQOto

— Amy Mount (@ASmallAMount) July 1, 2019

Similar concerns will be sparked by news Johnson is considering appointing Liz Truss and Dominic Raab as Business and Transport Secretaries, respectively. Noted environmental advocate Zac Goldsmith is also tipped for promotion as Environment Secretary, but campaigners remain fearful that Truss and Raab's past criticism of climate policies could hamper efforts to fast track a sufficiently ambitious net zero strategy. As Business Secretary Truss would have direct responsibility for meeting the UK's carbon budgets, and while she has recently voiced support for the new net zero goal she has also mocked the government's clean air rules and questioned the need to curb red meat consumption.

However, hopes are also building that a sea change is underway in the Conservative Party's engagement with climate change. MPs from across the party this weekend called on the new leader to remove many of the barriers to onshore wind farm development in the UK, after a poll commissioned by the Conservative Environment Network revealed overwhelming support for new projects amongst both Tory voters and the wider public. The poll is the latest in a string of surveys to reveal environmental action is now a top priority amongst voters.

Meanwhile, both the Hunt and Johnson campaign this morning moved to play down fears the UK would leave the EU without a deal in October after a flurry of warnings from business groups that crashing out of the bloc would prove disastrous for the economy. Johnson has said the UK must leave the EU at the end of October no matter what, while Hunt sparked criticism this weekend for declaring that businesses that go bust as a result of a no deal Brexit may have to make a necessary sacrifice to deliver Brexit. But both campaigns this morning stressed that their preferred option remained an improved exit deal with the EU - something Brussels has repeatedly said is not up for negotiation.

Whoever secures the keys to Number 10 business leaders are clinging to hopes a post-Brexit economic boost could be on the horizon, enabled by a coherent green withdrawal agreement, binding net zero target, and sufficiently ambitious decarbonisation policies. But at the same time Khan's warnings of a new era of climate 'delay' are not without foundation, especially when a 'no deal' Brexit economic crash, regulatory freeze, and new political era of Trump-inspired ultra-free market economics remains a distinct possibility.