Jeremy Corbyn has written to the British prime minister setting out five demands that need to be met to get Labour support for a Brexit deal.
Ahead of Theresa May's talks with EU leaders on Thursday, Mr Corbyn has insisted the PM needs to get Labour's priorities enshrined in the Political Declaration setting out future relations with the EU.
Mr Corbyn said securing in law the demands, which include joining a customs union, is the only way of achieving Labour support and uniting the country.
But the move has angered prominent pro-EU Labour figures who have accused Mr Corbyn of putting the Brexit policy agreed at Labour's conference "in the bin".
Downing Street confirmed that Mr Corbyn's letter had been received, and a spokeswoman added: "We will reply in due course. Our positions on many of the issues in the letter are well known. Our position on the backstop has not changed."
The spokeswoman said that no further meeting between Mrs May and the Labour leader to discuss Brexit had been scheduled, but added that "her door is open".
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The Labour leader has told the PM that just seeking modifications to the Northern Irish backstop proposals is not enough to win widespread backing and that she must change tack on key red lines.
In the letter to the PM, which follows their Brexit meeting last week, Mr Corbyn calls for a "permanent and comprehensive UK-wide customs union".
He states: "This would include alignment with the union customs code, a common external tariff and an agreement on commercial policy that includes a UK say on future EU trade deals."
Labour also wants close alignment with the single market "underpinned by shared institutions and obligations, with clear arrangements for dispute resolution".
Mr Corbyn calls for "dynamic alignment on rights and protections" so UK standards do not fall behind those of the EU, commitments on participation in EU agencies and funding programmes, and "unambiguous agreements on the detail of future security arrangements, including access to the European Arrest Warrant and vital shared databases".
He also calls for further talks with the PM "with the aim of securing a sensible agreement that can win the support of Parliament and bring the country together".
The letter does not mention previous Labour demands that any deal must deliver the "exact same benefits" as membership of the single market and customs union currently does.
Former shadow chancellor Chris Leslie attacked Mr Corbyn's move.
Mr Leslie tweeted: "Seriously? Offering to help Tory Govt enable #Brexit? It's not just Labour's conference policy in the bin.
"When the jobs go & revenues for services dry up as a result - Labour's leadership will have ZERO right to complain: they share responsibility."
Labour MP Phil Wilson, a supporter of the People's Vote campaign for a second referendum, said: "It was perhaps inevitable that at some point my party would publish something like this even though most Labour MPs, members and voters have long since worked out that there is no form of Brexit that can meet the promises made in 2016 or do anything but make people poorer.
"Our party conference agreed last year that if it couldn't get a general election it should explore other options including a new public vote.
"In the end there is only one way out of this mess for both our country and the Labour Party, a people's vote."
Shadow international trade secretary Barry Gardiner defended the shift in policy away from the six tests Labour originally set for a Brexit deal, saying the offer was made "in a spirit of co-operation and compromise".
"It's not about tests now," he told BBC Radio 4's Today programme. "What we are doing is saying we believe that these are the options that are available that would actually secure a majority in the House of Commons."
The Labour leader tells the PM that last month's rejection by MPs of her Withdrawal Agreement means a different approach is needed.
He writes: "Without changes to your negotiating red lines, we do not believe that simply seeking modifications to the existing backstop terms is a credible or sufficient response either to the scale of your defeat last month in Parliament, or the need for a deal with the EU that can bring the country together and protect jobs."
Responding to Mr Corbyn's letter, the chief executive of the Best for Britain campaign for a second referendum, Eloise Todd, said: "Labour has narrowed its Brexit options down, but the choice should not be between Brexit or putting a deal to the public - giving the people a say should be a matter of principle.
"What Labour must recognise is that whatever kind of Brexit Theresa May tries to negotiate, we are 50 days away from the current leave date with no clear plan, and suggestions to completely rethink the deal.
"This shows that nearly three years after the 2016 vote there is no consensus as to what Brexit should mean and it is therefore unthinkable not to consult the public - particularly if many of our future laws would be decided by 27 other countries with absolutely no power for the UK, as Corbyn's letter suggests.
"Labour must push the Tories harder to take no deal off the table and then give the public the final say on Brexit."
