The number of people suffering from excess "heat stress" at work is set to grow as climate change worsens, potentially costing the global economy $2.4tr every year from 2030 even if the world manages to stick within a 1.5C global warming pathway, the UN has warned.

An increase in heat stress at work linked to climate change is set to have a "massive impact" on global productivity and economic losses, the research suggests, particularly in the agriculture and construction sectors where workers are often outside.

The resulting lost output could equate to 80 million full-time jobs during 2030, or 2.2 per cent of total working hours worldwide, with the world's poorest countries set to be worst affected, the International Labour Organisation (ILO) estimates.

A report published on Monday by the UN agency calculates the total cost of these losses at $2.4tr per year based on a global temperature increase of just 1.5C above pre-industrial levels by 2100, which is the higher ambition target set out in the international Paris Agreement on climate change.

Catherine Saget, chief of unit in the ILO's research department and one of the report's lead authors, described the impact of heat stress on labour productivity as "a serious consequence of climate change".

"We can expect to see more inequality between low and high-income countries and worsening working conditions for the most vulnerable," she warned.

Heat stress is defined as generally occurring at temperatures above 35 degrees Celsius in places where there is high humidity, according to the report, which comes in the midst of a heatwave in Europe which saw temperatures in France soar above 45C - the highest ever recorded in the country. Scientists estimated that June's heatwave was made at least five - and possibly 100 - times more likely by climate change.

The report explains that excess heat at work is an occupational health risk that can in extreme cases lead to heatstroke, which can be fatal.

With around 940 million people working in agriculture around the world, farmers are set to be the worst hit by rising temperatures and could account for as much as 60 per cent of the estimated global working hours lost due to heat stress by 2030, the report states.

Construction is also likely to be "severely impacted", accounting for 19 per cent of global working hours lost from heat stress by the end of the next decade, it estimates.

Other high-risk sectors highlighted in the report include outdoor-based jobs in refuse collection, emergency services, transport, tourism, and sports, with southern Asian and Western African countries suffering the biggest productivity losses of around five per cent of working hours.

Saget said this would further exacerbate inequality between high and low income countries, leading to higher levels of poverty and displacement of people, and called on governments and businesses to improve climate resilience and labour safety standards to guard against the worst impacts.

"The impact of heat stress on labour productivity is a serious consequence of climate change, which adds to other adverse impacts such as changing rain patterns, raising sea levels and loss of biodiversity," she said.