Boris Johnson needs to “get off the sun lounger and start being a statesman” with less than three weeks to go before the Cop26 global climate change summit in Glasgow, Ed Miliband will say on Wednesday.

With Johnson on holiday in Spain, the shadow business secretary will say the UK is in danger of allowing “greenwashing” at Cop26, with insufficient pledges from countries across the world so far to make the summit a success.

Speaking at a Green Alliance event, the former Labour leader will ask why there is little public clarity on what Johnson is hoping to achieve at Cop26, with the government having failed to put forward its net zero strategy for the UK so far and stalling “miles away from where it needs to be” on international pledges.

Johnson has been making calls to world leaders on their Cop26 climate action from his holiday villa in Marbella, which was loaned to him by the environment minister and Tory peer Zac Goldsmith. He spoke to the crown prince of Saudi Arabia, Mohammed bin Salman, and Indian prime minister, Narendra Modi, on Monday.

However, Miliband said the government needed to put more effort into getting world leaders to promise big emissions reductions before the summit, which it is hosting for two weeks from 31 October.

He said there was an “undeniable and frightening maths of Glasgow” that less than four gigatonnes of emissions were estimated to have been pledged so far, however the reduction needed was closer to 28 gigatonnes.

“We cannot let Cop26 be the greenwash summit,” he said. “It’s time for the prime minister to get off his sun lounger, be a statesman and make Glasgow the success we need it to be. This summit must succeed. It still can.

“But we need a step-change in action from our government and governments across the world. Above all, finally, at the 11th hour, the prime minister must treat this summit with the seriousness which it deserves.”

He said the central task of government was to close the gap between the aggregated country-by-country targets and the high-level ambition of the Paris Cop summit.

Alok Sharma, the Cop26 president, has been meeting politicians all over the world in an attempt to thrash out ambitious pledges – but the leader of the Cop host country often has to get involved in leading preparations and the talks themselves.

Miliband said the government so far had been “at best bystanders and at worst, contributors to global inaction”.

He added: “When trust between developing and developed countries is the key to success, and we need to persuade others to step up on climate finance, the UK took the disastrous decision to cut the aid budget, the only G7 country to do so.

“When we are telling every major emitter they must act, the UK has done a trade deal with Australia allowing them to delete Paris temperature commitments from the text. When we have rightly made powering past coal a focus of our presidency, at the very same time the government has flirted with a new coalmine in Cumbria.”

He also said the energy crisis was already hitting families and firms with higher prices, and a failure to deliver at Cop26 would mean a further crisis for the economy in the decades ahead.

Referring to the row between Kwasi Kwarteng, the business secretary, and Rishi Sunak, the chancellor, over whether to bail out firms hit by high energy costs, Miliband added: “Ministers are turning on each other when they should be turning outwards to engage with industry and take action by intervening. We can’t sit back and watch whole British industries go to the wall.”

A government spokesperson said tackling climate change was a personal international priority for Johnson. “He has been clear that Cop26 must be the moment that every country, and every part of society, embraces their responsibility to protect our shared future and is looking forward to meeting with leaders – from big emitters to climate vulnerable nations – to make sure Cop26 counts.”