David Cameron has come under further pressure to produce measures aimed at boosting the powers of Westminster over Brussels as the Prime Minister continued attempting to build support for his reform demands.
Mr Cameron has said he wants to put "beyond doubt" the sovereignty of the UK Parliament, but a Tory MP leading a Commons debate claimed he was "side-stepping" the issue.
Meanwhile, another senior Tory ridiculed Mr Cameron's "emergency brake" proposal on curbing benefits to EU migrants and claimed the "red card" allowing parliaments to block laws from Brussels was a "desperate attempt to put lipstick on a pig".
The Prime Minister was meeting key players from the EU in London, holding talks with European Council president Donald Tusk, Slovak PM Robert Fico, Greek PM Alexis Tsipras, Swedish PM Stefan Lofven and Belgian PM Charles Michel, in the margins of the Syria donors' conference in London.
He will also meet European Parliament president Martin Schulz amid continued speculation about the role MEPs could play in deciding the fate of any deal.
Tory MP John Baron, who led calls in the Commons for Mr Cameron to strengthen the role of Parliament in stopping the flow of laws from Brussels, mocked the "red card" which would allow measures from Brussels to be blocked, but only if 55% of EU national parliaments agree.
He said: "The so-called 'red card' system is nothing more than a washed-out lottery ticket. It would be like a football referee getting out his 'fraction' of a red card, only then having to consult with 14 other officials before deciding what to do - by which time the game is over."
Meanwhile, former Tory leadership contender David Davis ridiculed both the "red card" and the "emergency brake" - two key measures contained in the proposed deal which Mr Cameron hopes to strike with EU leaders at a summit on February 18-19.
In a speech in central London, the Haltemprice and Howden MP said: "The 'red card' system only operates on draft laws, only works if there is a 'subsidiarity' argument, and needs the agreement 55% of EU Parliaments.
"This is the same as the old 'yellow card' system, that was also unworkable and which the Government has previously claimed is too difficult to satisfy.
"Since its introduction in 2009, it has only been used twice, one of those times being the proposal to create an EU public prosecutor's office, which went ahead anyway.
"The Government's feeble rebranding from yellow to red is a desperate attempt to put lipstick on a pig."
The brake mechanism, allowing countries to block in-work benefits to migrants if their welfare systems or public services were placed under strain, would "have no impact whatsoever", Mr Davis said.
"If my car's brakes worked in a similar manner, then there would be an almighty crash."
Former Home Secretary Alan Johnson, the leader of Labour's campaign to remain in the EU, said the emergency brake would have no impact on immigration as in-work benefits were not a factor in drawing people to the UK.
Asked whether Mr Cameron believes the proposed "emergency brake" will cut immigration, a Downing Street spokesman said that migrant families were currently able to claim an average of £6,000 a year in tax credits and some 10,000 were claiming £10,000 or more.
Mr Tusk's reform package "acknowledges what the Prime Minister has been saying for a long time, that the benefits system is an unnatural draw for EU nationals", said the spokesman. "I think common sense would tell us that reducing the financial incentive will reduce that pull factor."
Playing down fears of a lengthy delay in implementing the measure, the Number 10 spokesman highlighted comments from Manfred Weber, the leader of the European Parliament's largest European People's Party grouping, that legislation to put the emergency brake mechanism in place could be completed within one to three months after a referendum.
The spokesman was unable to say whether migrants from wealthier EU states, such as Luxembourg, might end up receiving more generous child benefit payments than their British neighbours if they were brought in line with levels in their home country.
Asked if that was a possible outcome, the spokesman said that details were still being worked out, adding: "We have draft proposals on the table. There are negotiations that are ongoing."