Major supporters of Brexit have urged the prime minister not to resolve the UK "divorce bill" unless the EU accepts a number of conditions.
The demands of the Leave Leave Leave group include the finalization of the jurisdiction of the European Court of Justice at the time when the UK leaves the EU in 2019.
It arrives with talks at a key moment before a key summit later this month.
Meanwhile, a minister suggested that Brexit might not happen at all unless Theresa May is backing the negotiations.
Health Secretary Jeremy Hunt told ITV: "The choice we face now is not between this Brexit and that Brexit, if we do not support Theresa May we will not have Brexit, and she is doing an incredibly challenging job Surprisingly well. "
Ms. May will travel to Brussels on Monday to talk with European Commission President Jean-Claude Juncker, hoping the EU will agree that "enough progress" has been made so far to start the trade talks.  Leave Means Leave, who campaigns for "a clean Brexit," wrote an open letter to the PM setting out certain conditions that the EU should agree upon before the UK badumes additional financial commitments.
What is it that keeps?
The talks so far have focused on what happens to the rights of citizens after Brexit, the amount of money the UK will pay and the future of the Irish border.
In the "divorce bill", it is understood the United Kingdom has recently increased its offer, which could be worth up to 50,000 million euros (£ 44bn).
There has also been a lot of focus on the issue of Ireland in recent weeks, with the Irish government looking for more information on the "frictionless border" "The UK wants to be established so that customs controls are not needed."  Will Ireland's demands delay Brexit?
The EU will only move forward to talk about future issues such as trade when sufficient progress has been made on these issues, and must decide if this happened at a summit on 14 and 15 December 19
What is Leave Means Leave?
The campaign group is described as the "Campaign for a Clean Brexit."
Signatories to their letter to the Prime Minister include the former Minister of the Tory Cabinet. s Owen Paterson, John Redwood and Lord Lawson and former Minister Brexit David Jones.
Backbench deputies Jacob Rees-Mogg (Conservative) and Labor Graham Stringer have signed it, as have several economists and lawmakers. s.
Ex-conservative Lord Howard, who did not sign the letter, told the BBC Sunday Politics that he shared the "aspirations" it contained.
What they want
The letter says that the EU has not equaled "patience and goodwill" Theresa Mayo has shown so far and says that the UK should not make any "additional financial commitment" until it accepts certain conditions.
As of March 2019, they say:
- There must be an agreement "in principle" for a current trade agreement
- The current free movement of EU citizens in the United Kingdom should be terminated
- No new EU regulations should be applied in the United Kingdom
- The European Court of Justice should "no longer have any jurisdiction" in the United Kingdom
The United Kingdom should be ready to return to the conditions of the Organization World Trade if a future free trade agreement with the EU is not guaranteed, they add.
Speaking on the Andrew Marr Show of the BBC, Rees-Mogg said if the United Kingdom remains subject to European judgments after March 2019 "we will stay in the European Union".
What the government says
Theresa May has promised that Brexit will terminate the jurisdiction of the European Court of Justice in the United Kingdom.
But she has suggested her mandate could continue during an "implementation period" after March 2019, which would not satisfy the demands of Leave Means Leave.
On the issue of immigration, ministers have said that free movement should end EU membership in the United Kingdom but have promised companies "will be avoided in terms of employing foreign workers."
Meanwhile, there have been further warnings on the subject of the EU court from former Court of Appeals Judge Sir Richard Aitkens and former Conservative leader Iain Duncan Smith.
The Sunday Telegraph reports that Sir Richard, chairman of the Lawyers for Britain group, has written to Ms. May saying that giving TJJ exclusive jurisdiction over the rights of EU citizens would be "tantamount to reversing the outcome of the 2016 referendum."  In a separate article in the Telegraph, Duncan Smith warns that the plan would be "quite unacceptable" as it would put the United Kingdom in the position of "cede power to a foreign court".