UK Labour leader Keir Starmer has come under fire from all sides after the party suffered a crushing defeat in the Hartlepool by-election.

In a stunning result, the Conservatives took the seat – which had been Labour-held since it was formed in 1974 – with a majority of 6,940. Boris Johnson hailed the outcome as “very encouraging” as the Conservatives appeared poised to make further gains in council elections across England.

Allies of the Labour leader acknowledged that it was an “absolutely shattering” blow as another pillar in the party’s once impregnable “red wall” crumbled. They insisted however that Mr Starmer would not be deflected from the course he had set out when he succeeded Jeremy Corbyn a little over a year ago.

However, there were calls from both the left and the right of the party for an urgent change of direction if they were to stand any chance of regaining power at the next general election.

On the left, former shadow home secretary Diane Abbott said it was not possible to blame Mr Corbyn for the setback and said Starmer needed to “think again” about his strategy. Mr Corbyn himself said the results showed a loss of hope. “We must offer a bolder vision to transform people’s lives and give them the confidence to strive for a more equal world,” he said.

Former shadow chancellor John McDonnell said the party had gone into the by-election “almost policy-less” and called for a return to a “real grassroots campaign”.

Unite general secretary Len McCluskey said the loss was “staggering and worrying” and clear evidence that the strategy of the last year had not worked.

But there was criticism too from the right, including the Blairite former cabinet minister Lord Adonis. “Clearly the public isn’t persuaded that Labour has either the leadership or the policies or the critique or the energy and the dynamism in terms of its view and we all need to consider that,” he said.

The warning came as early results in council contests appeared to show voters deserting Labour. With results in from 17 out of 143 councils, the Conservatives had gained a total of 56 seats while Labour had lost 57.

In Hartlepool, Conservative candidate Jill Mortimer gained 15,529 votes, with Labour’s Dr Paul Williams trailing on 8,589.​​