Firing the warning at any potential leadership rivals, he added: “We hear suggestions that this is being seriously contemplated by some at the very top of the Government.

“Moreover, we hear that it appears to be the aim of people who now have designs on the PM’s job.

“Andy Burnham has declared how he wants to see us back in the EU in his lifetime, while Wes Streeting wants us to have a deeper trading relationship with Europe.”

A row erupted after Health Secretary Wes Streeting called for a “deeper trading relationship with the EU” – a move which was interpreted as both a call to rejoin the Customs Union and an attempt to gain the support of Europhile Labour MPs ahead of a leadership bid against the Prime Minister.

Manchester Mayor Andy Burnham, a leadership contender, has also stated that he hopes the UK will rejoin the EU in his lifetime.

Speaking to the Mail on Sunday (MoS), Mr Carden added: “The European Union (EU) is… a low-growth bloc with a declining share of global GDP. Joining a Customs Union would mean scrapping our post-Brexit trade deals with places that are the 21st-century growth centres, such as the US and India.

“Shifting closer to Brussels would mean giving up our hard-won national freedoms.

“I urge Sir Keir not to seek to tie the UK into a new customs union or any other such arrangement which may carry a disguised name but amount to the same thing.”

Labour MP Graham Stringer told how Sir Keir would face “a rebellion of scores of Labour MPs if he sought to take us back into any sort of Customs Union”.

He told the MoS that “it will lead to civil war inside Labour”. He added: “Starmer’s Parliamentary colleagues know they stood on a clear manifesto pledge to do no such thing.

“It would be especially difficult for Labour MPs in the so-called Red Wall to support such a flagrant manifesto breach, as many have Reform breathing down their necks. They would be committing electoral suicide.”