Labour's support of Britain's continued membership of the EU risks pushing millions of the party's "core voters" into the arms of Ukip, senior Labour backbencher Frank Field has said.
Mr Field said Jeremy Corbyn's decision to support the Remain campaign could be the "second longest suicide note" in the party's history after the infamous 1983 election manifesto, which was criticised for policies perceived as outlandish.
"Since we had Tony Blair's mega election victory in 1997, we have lost 4.2 million voters," Mr Field told the BBC Radio 4 Today programme. "Many of those - maybe a million at the last election - went to Ukip.
"They went because they thought we had a leadership which didn't represent their views about the nature of the country they wished to live in - the country they were born in.
"I think, therefore, the question about borders and control over borders is crucial to this referendum vote."
Mr Field said he believes Mr Corbyn has only put his support behind the Remain campaign because he feared a challenge to his leadership if he publicly backed Leave.