Labour will vote down any attempts to force the country into a “blind Brexit”, Sir Keir Starmer has said.
The shadow Brexit secretary has warned government counterpart Dominic Raab the Opposition will reject any “vague” plans put before Parliament.
It comes as Labour’s London mayor heaped pressure on Jeremy Corbyn by calling for a fresh referendum.
Sadiq Khan said voters should have the final say on what happens next.
In an article for the Observer, he said the UK faces either a bad deal or no deal.
“They are both incredibly risky and I don’t believe Theresa May has the mandate to gamble so flagrantly with the British economy and people’s livelihoods,” he wrote.
Mr Khan said voters now need to be given a new referendum.
“This means a public vote on any Brexit deal obtained by the Government, or a vote on a no-deal Brexit if one is not secured, alongside the option of staying in the EU,” he wrote.
Mr Corbyn will face intense pressure at Labour’s autumn conference later this month to back a fresh Brexit poll.
Mrs May, meanwhile, is facing strong opposition to her Chequers blueprint for leaving the EU from hardline Tory Brexiteers.
Without support from at least some opposition MPs, the Government is likely to struggle to get any agreement through Parliament.
A loosely-worded deal could be struck that would be able to be sold to all sides.
Sir Keir, however, said the Government must meet its key Brexit tests, which include delivering the “exact same benefits” as the UK currently has as members of the single market and customs union, to win support from Labour.
In a letter to Mr Raab published by The Sunday Times, he added: “But, let me be equally clear: a vague political declaration would not meet those tests. Labour will not – and cannot – vote for a blind Brexit.”