Pro-Brexit Tory MPs set out demands in letter to Theresa May
More than 60 Brexit-supporting Tory MPs have written to Theresa May to insist the UK make a clean break with the EU.
The MPs say the UK must not be stopped from negotiating trade deals with other countries, once it leaves the EU, and must gain full "regulatory autonomy".
The letter was sent by the European Research Group of Tory backbenchers, headed by Jacob Rees-Mogg.
Opposition critics accused the PM of being "too weak" to confront the "hard Brexiteers" in her own party.
The letter, signed by 62 MPs including several ex-ministers and former Conservative Party leader Iain Duncan Smith, includes a number of "suggestions" for securing a successful Brexit.
It backs the PM's vision for Brexit, as set out in her Lancaster House speech in January 2017, including leaving the customs union and single market.
But it says she "must" ensure the UK can change its laws without authorisation from the EU from the moment it leaves in March 2019.
The MPs also insist the UK must be "free to start its own trade negotiations" during the planned two-year transition period that would kick in after Brexit day.
And it urges the UK government to resist EU efforts to set the timetable for Brexit negotiations.
"The UK should negotiate as an equal partner. Ministers may not want or be able to accept the EU's timing and mandates as fixed, and should be able to set out alternative terms including, for example, building an agreement based on our World Trade Organisation membership instead," says the letter.
It comes ahead of a crunch cabinet meeting on Thursday to thrash out an agreement on how to proceed in negotiations with the EU.
The government is also set to publish its its response to the EU's proposals for how the two-year transition period after Brexit should work later on Wednesday.
The UK wants a mechanism to object to new European legislation introduced during the implementation phase.
The prime minister had suggested that EU nationals who arrive during the transition period should not have the right to settle permanently in the UK.
Why the letter to May matters
Analysis by BBC political editor Laura Kuenssberg
With no majority, she knows that she needs to keep the dozens and dozens of Brexit-backing Tory MPs broadly with her for her own government's survival.
They have accepted some shifts from the government that they used to find intolerable - a Brexit departure lounge of a couple of years rather than a sharp exit, and a bill of tens of billions.
But they are not, as this letter makes clear, up for swallowing many more compromises when it comes to getting trade deals done immediately after Brexit.
Labour claimed the MPs' letter "exposes the deep divisions that run through the heart of this Tory government".
Shadow Brexit minister Paul Blomfield said: "It is clearer than ever that Theresa May cannot deliver the Brexit deal Britain needs.
"She is too weak to face down the fanatics in her own party and to deliver a final deal that protects jobs and the economy."
The SNP's Stephen Gethins said: "It is clear from this list of demands that the Tories don't want either a transition deal or a future relationship with the EU."
Former Lib Dem leader Tim Farron said it appeared the prime minister had "one arm tied behind her back by the Tory militants who are now nakedly acting like a party within a party".
Rival forecast published
In other Brexit news, a group of pro-Leave economists claim to have "comprehensively debunked" leaked government forecasts predicting a hit to the UK economy outside the EU.
The Whitehall forecasts - which predicted lower growth across the UK as a result of Brexit - sparked a row when they emerged last month.
Now Economists for Free Trade have published their "alternative" calculations, saying the civil servants' version ignored what they said were the "clear objectives" set out by Mrs May of free trade with Europe and the rest of the world.
Using government models, this would suggest a 2% rise in GDP over 15 years, the group claimed.
Mr Duncan Smith said the report "deserves to be taken very seriously".
Mrs May is meeting Dutch Prime Minister Mark Rutte for talks in Downing Street and is expected to update him on Brexit progress.