Labour said the target of keeping global warming within 1.5C was in “intensive care” following the agreement reached by world leaders at the end of the Glasgow summit.
The opposition claimed the prime minister had left his Cop26 president in a weak position, because of the government’s decision to cut overseas aid and failure to stop the Cambo oilfield project.
Deputy leader Angela Rayner also criticised Mr Johnson’s failure to be there in the conference’s final hours to help Mr Sharma push China and India to make a stronger commitment to end dependence on coal.
“Boris Johnson could have done more to be there, and to make sure that in that room at those vital opportunities that [he would] be able sway the opinion,” Ms Rayner told the BBC’s The Andrew Marr Show.
She added: “We saw Alok Sharma doing his utmost. But Boris Johnson has undermined some of our efforts by using fossil fuels, the investment in the that, by the cutting of overseas aid. There’s much more we can do to set an example globally.”
In a dramatic last-minute intervention at the Glasgow summit, India and China were able to change the wording of the final deal so coal power generation would be “phased down” rather than “phased out”.
Mr Sharma – who appeared to break down briefly as he apologised to delegates and campaigners for the late change – claimed on Sunday that the weakened deal still represented progress. “We kept 1.5 in reach,” he said.
Labour’s shadow Ed Miliband told told Sky News’s Trevor Phillips on Sunday that the goal of keeping 1.5C alive had been left in “intensive care” following the pact agreed in Glasgow.
He also accused Mr Johnson’s government of “undermining” Mr Sharma. “I have nothing but praise for Alok Sharma. But I’m afraid the rest of the government didn’t help him and undermined him with decisions like cutting overseas aid,” said Mr Miliband.
The Labour frontbencher added: “Because we were then saying to other countries, ‘Please step up on climate finance’, when we were stepping back on aid to poorer countries.”
On the potential for new drilling in then Cambo oilfield drilling near Shetland and a coal production facility in Cumbria, Mr Miliband said it would be “total hypocrisy” for the government to allow them to go ahead.
“Frankly it looks like total hypocrisy when we are trying to persuade other countries to act,” he said. “We do need to show we can act and we need to show we are not facing both ways as a country … That’s been a problem with this government.”
Mr Johnson and Mr Sharma will be grilled over the climate deal agreed in Glasgow at a press conference later on Sunday. The pair will be answering questions about the global pact at Downing Street at 5pm.