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Eight pro-Brexit lawyers, including seven Tory and DUP MPs, have outlined the key terms Theresa May must deliver to secure their support for her deal to leave the European Union.
They want to examine any agreement Attorney General Geoffrey Cox reaches with Brussels over the Northern Ireland backstop - to ensure it is temporary.
The backstop is designed to avoid a hard border with Ireland after Brexit.
It comes as a senior Brexiteer pointed to MPs' growing mood for compromise.
Writing in the Mail on Sunday, Sir Graham Brady urged other backbenchers to "pull together behind the prime minister" when the "right compromise" could be offered.
Our political correspondent Iain Watson said the verdict from the group of lawyers was significant because one of them was the DUP's Westminster leader Nigel Dodds, and another was the former Brexit secretary Dominic Raab.
"If they pronounce that Geoffrey Cox has made substantial progress in Brussels, then they are likely to persuade other long-standing leave campaigners that they can back the Prime Minister's deal," he said.
'Breakthrough is near'
The group, most of whom are from the Brexit-supporting European Research Group (ERG), have drawn up three tests they want to be met by the government, including a legally binding "mechanism" to escape the backstop.
In an interview with the Sunday Times, one of the lawyers - Michael Tomlinson - said they must see any concession from Brussels "in black and white" and in plenty of time before the PM asks Parliament to vote on her revised deal.
Meanwhile, senior Conservative backbencher Sir Graham Brady indicated a breakthrough could be on the cards.
Sir Graham said: "This is not a time to make the best the enemy of the good, and most MPs are in a mood to compromise, but the danger of this backstop becoming permanent is a real one and it has to be tackled.
"My conversations with senior diplomats and politicians from across Europe have given me cause for optimism that a breakthrough is near."