President willing to step down – but only in exchange for his country’s membership of Nato.

On Sunday, Lord Mandelson urged Mr Zelensky to make up with Donald Trump and support his peace initiative, saying it is the “only show in town”.

Britain’s ambassador to the US called on the Ukrainian president to give his “unconditional backing” to US plans to bring an end to the three-year conflict.

He called on Ukraine to be the first to commit to a ceasefire and warned that if the US president’s peace initiative “goes wrong”, then Britain would have to support Ukraine in the event of a Russian attack.

Lord Mandelson was hand-picked by Sir Keir to take over as Britain’s ambassador to the US, as part of his plans to act as a bridge to Mr Trump.

However, there were fears he would not be approved by the new US administration after one of Trump’s allies called him a “moron”.

Lord Mandelson said he wanted to see Mr Zelensky and Mr Trump “getting back on the same page”.

“We need a very radical reset and it has to consist of the United States and Ukraine getting back on the same page, and president Zelensky giving his unequivocal backing to the initiative that president Trump is taking to end the war and to bring a just and lasting peace to Ukraine,” he told ABC.

“And the Europeans too, they need to back the calls for a ceasefire; and by the way I think that Ukraine should be the first to commit to a ceasefire and defy the Russians to follow.”