As a battle rages within Labour over its stance in a second Brexit referendum, the Shadow Foreign Secretary said she is “fearful” of another five years of Boris Johnson in Downing Street unless Jeremy Corbyn abandoned plans to remain neutral on leaving the EU in a snap general election.
Ms Thornberry revealed internal polling carried out by affiliated trade unions and campaign groups shows Labour would lose 30% of its 2017 vote across the UK to pro-Remain parties, particularly the Liberal Democrats.
The polling suggests that under its current policy of putting a new Brexit deal to a referendum with Remain on the ballot paper, but with Labour’s stance unclear in a general election, only 58.7% of 2017 Labour voters would stick with the party.
The Lib Dems would take 19% of the 2017 Labour vote, with 7.4% going to the Greens, 3.5% to the SNP, and 0.7% to Plaid Cymru.
That compares with 7% going to the Brexit Party and 4% to the Conservatives.
Ms Thornberry warned Labour would only be given a hearing on its social and economic policies in a snap election if it was clear on Brexit.
“My concern is that unless we’re clear about our Brexit policy,we may not get off first base because [the media] will constantly be asking us what our Brexit policy is,” she said.
“I do think we suffered in the European elections by not being sufficiently clear, and my view is that we are not being sufficiently clear now.”
The Shadow Foreign Secretary went on: “My concern is that unless we do put forward a full programme [on Brexit], I’m concerned that we will end up with Boris Johnson being back as prime minister for another five years.”
Ms Thornberry described Labour’s position as “a little bit like that scene in Star Wars, where they’re in the garbage crusher and the walls are coming in on either side.
“You don’t just stand there and say ‘I'm right’, you have to get out of there. If we don’t, I’m fearful for the result of the general election.”
Her comments follow a call from Scottish Labour leader Richard Leonard for the UK party to follow policy set north of the border and in Wales, backing Remain in the event of a second EU referendum.
Mr Corbyn is under pressure ahead of a vote of members in Brighton tomorrow, after he said he would "go along with whatever decision the party comes to" at a special conference to decide Labour’s stance in a referendum.
“In a polarised situation like this, it's absolutely right to keep the country together, which is what a second referendum does in my view, but we’ve got to be honest about who we are - and we’re internationalists and Europeans,” Ms Thornberry added.
“I know that the Scottish Labour Party agreed with me, and I know that the Welsh Labour Party agree with me.”
Pressure was also heaped on the Labour leader by the London Mayor, Sadiq Khan, who told a fringe event in Brighton: "Staying neutral in the face of the biggest economic and social threat to our country for decades is simply not an option.”