The shadow foreign secretary said “all options remain on the table” if Labour fails to force a general election by voting against the Brexit deal Theresa May still hopes to strike within the next few days.
Mr Corbyn has angered Labour MPs and supporters by arguing Brexit cannot be stopped – and by going on to reject the growing calls for a fresh public vote.
But Ms Thornberry said: “If we don’t have a general election then, yes, of course all the options remain on the table and we would campaign for there to be a people’s vote – but there are several stages before we get that.”
Speaking to the BBC, she condemned the prime minister for failing to offer a true “meaningful vote” on the Brexit deal, by insisting the only alternative to rejection is to crash out of the EU with no agreement at all.
Calling that a choice to either “fall off the cliff or get on this bridge to nowhere”, Ms Thornberry said: “That’s not a meaningful vote – that’s not an injection of democracy.
“So we say, if you’re not going to give us that then we refuse to play that sort of game” – confirming Labour would vote against the deal and try to force an election.
Pro-EU backbenchers accused Mr Corbyn of a “dereliction of duty” over his stance and warned he would never be forgiven for ignoring the overwhelming wishes of Labour members.
But he doubled down on Saturday, when asked if he could agree with the call by former Tory minister Jo Johnson for a fresh referendum, replying: “Not really, no.
“The referendum took place. The issue now has to be how we bring people together, bring people together around the principles of our economy, our rights and that we don't turn this country into some kind of offshore tax haven on the lines that Donald Trump might want us to.”
On Friday, he told the German newspaper Der Spiegel: “We can't stop it. The referendum took place. Article 50 has been triggered. What we can do is recognise the reasons why people voted Leave.”
On the Andrew Marr Show, Ms Thornberry attacked the prime minister for asking MPs to choose between “the devil and the deep blue sea”.
“You cannot simply come to the House of Commons with a bit of nonsense that makes no sense,” she said.
“You cannot expect the Labour Party to save you from your own backbenchers who are saying this deal makes no sense – and everybody knows it doesn't make sense.”
Asked if the party had had a “serious conversation” with a senior EU official who had indicated that, she replied: “Oh no, of course not.
“But what we've had are discussions and they know what it is that we want, and they know we are democrats and if they were in our position they'd be trying to negotiate exactly the same way as we are.”
Ms Thornberry rejected the suggestion that Labour was indulging in the same fantasy politics as leading Brexiteers, insisting: “It is not a fantasy prospectus.”
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