Theresa May is attempting to prevent a Tory rebellion over Brexit - as Number 10 rejected a claim she was preparing to approve a divorce bill of up to £50 billion.
Brexit Secretary David Davis also dismissed the Sunday Times report that the Whitehall plans will see the UK pay between £7 billion and £17 billion a year to Brussels for three years after Brexit.
The newspaper said Mrs May would approve the bill after the Tory conference in October and said it was £20 billion higher than hoped, as a result of her weakened domestic position since the General Election.
Number 10 sources said the claim was "not true" before Mr Davis told the BBC's Andrew Marr Show the exit figure was "nonsense".
He said there was strictly "no enforceable" legal obligation to pay a bill but said there was a "moral ... or political" obligation that the UK would fulfill.
The financial settlement was the subject of a bitter row between Mr Davis and Brussels' chief negotiator Michel Barnier during the latest round of negotiations on Britain's EU withdrawal.
The payment dispute came as Mrs May's allies issued a warning to would-be rebels ahead of the first Commons votes on the Brexit legislation.
They claim any attempt to water down the so-called Repeal Bill will risk putting Labour leader Jeremy Corbyn in power.
The Bill - officially called the European Union (Withdrawal) Bill - repeals the 1972 European Communities Act which took Britain into the EU.
It is designed to transfer EU law into British law so the same rules apply before and after Brexit, while giving parliaments and assemblies in Westminster, Edinburgh, Belfast and Cardiff the power to drop or change them in the future.
Mrs May has already said it will give legal certainty to people and businesses and insisted there will be "proper Parliamentary scrutiny".
In July the Government confirmed it would not incorporate the European Charter of Fundamental Rights into British law.
The legislation will be debated on Thursday after MPs return to Westminster following a summer break in which some have been plotting their tactics for the Brexit battles ahead.
However, pro-EU Tories have claimed the whipping operation to ensure they backed the legislation could backfire and potentially damage the Prime Minister's already weakened leadership.
"Since Parliament broke for summer the Government has been working hard to deliver a successful Brexit," Mrs May said.
She continued: "The series of papers we are publishing have provided further detail on the deep and special partnership we want, we've had frank negotiations with the Commission, and we've travelled the globe to establish the trading relationships of the future.
"Now it is time for Parliament to play its part.
"The Repeal Bill delivers the result of the referendum by ending the direct role of the EU in UK law, but it is also the single most important step we can take to prevent a cliff-edge for people and businesses, because it transfers laws and provides legal continuity.
"We have made time for proper Parliamentary scrutiny of Brexit legislation and welcome the contributions of MPs from across the House.
"But for us to grasp the great prize ahead of us, that contribution must fit with our shared aim: to help Britain make a success of Brexit and become that great global country we know we can be."
The Prime Minister's de facto deputy, Damian Green, the First Secretary of State, used a Sunday Telegraph article to warn Tories not to derail the legislation.
"Starting the new parliamentary session with the Withdrawal Bill shows that it is now the job of all MPs, including my former colleagues on the Stronger In campaign, to respect the will of the people and get the best possible deal for Britain," he wrote.
"No Conservative wants a bad Brexit deal, or to do anything that increases the threat of a Corbyn government."
But outspoken pro-Remain former minister Anna Soubry responded angrily to attempts to try to stop Tory MPs tabling or supporting amendments to the legislation, telling the Observer: "Any suggestion that this is in any way treacherous or supporting Jeremy Corbyn is outrageous.
"It amounts to a trouncing of democracy and people will not accept it."
She added: "People will be very alarmed. It will all backfire on them."