Britain and other European nations must be ready to take over Nato if Donald Trump carries out US threats to withdraw from the organisation.

They must replace American military aid to Ukraine scrapped by Mr Trump and make sure Russian leader Vladimir Putin does not win the conflict.

That was the powerful message delivered on Saturday by former Conservative defence secretary Sir Ben Wallace.

“We are witnessing a new era where we cannot take for granted US security guarantees,” said Sir Ben.

He accused President Trump of trying to “bully” Ukraine and “suffocate” its hopes of defeating Russia.

“We aren’t going to let Putin win,” Sir Ben told the BBC. “The Americans can do what they want, but what Trump may discover… is that the Ukrainians are tough people.

“Putin is not going to stop and President Trump is naive to think so.”

Sir Ben said Mr Trump’s message to Putin appeared to be: “I will bully Ukraine and give hope to Russia.”

He continued: “That is the totally wrong message to send if you want to push Russia back and make Putin think that he had better come to the table.

“Donald Trump is suffocating the hope that Putin can be fought to a standstill and brought to the table.”

Sir Ben said that if Ukraine carries on its resistance for another 18 months, “the Russian economy will collapse”.

Ukraine could still defeat Russia, but only “if we all step up and substitute what the Americans have taken away or show proper European resolve, including the UK, that we ain’t going anywhere”.

Britain and Europe had to face up to the fact that it was possible the US could carry out threats by vice-president JD Vance and others to leave Nato altogether, said Sir Ben.

Britain and Europe could survive withot US support, he argued, but it would mean spending a lot more money on defence.

He added: “It’s not the end of the world if they pull out of Nato. We have the will in Europe and the money, if we choose to, to fix our own security and defence.”

It was partly a question of “resolve”, said Sir Ben: “Do we want to do this, see through the defence of Ukraine, make sure we are less reliant on a less dependable US by investing in our own capabilities?

“We have the capability, if we have to, to replace that. It’s not like if they pull out we have no options at all if we want to, if we believe in ourselves and are prepared to put our hands in our pocket.”