Johnson will host more than 120 leaders in Glasgow early next week, before the UN meeting spends a fortnight tackling the grinding work of deciding how to limit temperature rises to 1.5 degrees Celsius.

"It is of course correct that COP26 is both unbelievably important for our planet but also very difficult, and it's in the balance," he told parliament, having played down expectations in recent days.

Ahead of a separate G20 summit this weekend in Rome, Johnson said 17 of its 20 members had now produced plans to curb their carbon emissions -- although China and India remain significant outliers.

Under the UK's presidency of COP this year, the premier said, 80 percent of the global economy had committed to net zero emissions by 2050, up from 30 percent before.

"And every day as I talk to international leaders, we hear further commitments to make those solid commitments that the world will need," he said, noting recent commitments by Australia, Indonesia and Russia.

"Whether it is enough, I'm afraid it is too early to say," Johnson added.

Ed Miliband, business spokesman for the opposition Labour party, attacked Johnson for sending the wrong message ahead of COP26 by cutting Britain's aid budget.

Financial assistance from richer countries for developing nations to fight climate change is one of the pivotal issues coming up in Glasgow.

"COP26 is not a glorified photo opportunity," Miliband told Johnson, demanding "climate delivery, not climate delay".

Miliband was standing in for Labour leader Keir Starmer at the weekly session of prime minister's questions, after Starmer tested positive for Covid-19.