European Parliament Vice-President Alexander Graf Lambsdorff has attacked UK Prime Minister David Cameron's attempt to alter the terms of Britain's EU membership ahead of the upcoming in/out referendum.
Under the terms of the proposal, Britain will be allowed to employ an 'emergency brake' to restrict new EU migrants from having full access to benefits for up to four years.
This, however, is not the outright four-year ban Cameron had initially tried to secure.
Even so, Lambsdorff, an ALDE group member, said his renegotiation package went "too far" and will face major opposition among MEPs.
The German deputy warns he will "certainly not agree" to changing fundamental EU rules in order to back Cameron's proposed 'emergency brake' on migrants claiming in-work benefits in the UK.
The consent of Parliament would be required for a treaty change before an emergency brake could come into force.
Lambsdorff casts serious doubt on whether this will be approved by MEPs.
Lambsdorff also reportedly described Cameron's package of reforms as "nothing more than a deal that has been hammered out down the local bazaar".
He also believes that while he would prefer the UK to stay in the EU, the outcome of the 23 June referendum is on a "knife edge" and too hard to call.
Parliament's group leaders welcomed the deal reached at the Council summit earlier this year on the UK's EU membership renegotiation demands, urging British voters to choose to remain part of the EU.
MEPs will assess Cameron's deal and vote on key aspects, such as the contentious emergency brake on welfare benefits, before it can become legally binding, should the UK vote to remain in the EU.
Former ALDE group MEP Andrew Duff backed Lambsdorff, telling this website, "He's perfectly within his rights as an MEP to hold his opinion. The Cameron deal on 19 February was binding only on his 27 colleagues in the European Council. Implementing the deal will be very complex, very controversial and very long - and not just in the European Parliament."
Lambsdorff's comments, however, were immediately criticised by Ukip deputy leader Paul Nuttall, who said: "This intervention from a Vice-President of the European Parliament leaves Cameron's proposal in tatters.
"It was just a proposal with the important changes dependent on the future approval of MEPs many of whom have already said they will oppose it.
"Given that his pro-EU propaganda leaflets contains messages about his little deal, I would demand that Cameron withdraw his lying leaflets immediately."
Lambsdorff comments are 'nonsense', says Tory MEP
Elsewhere, ECR group member Charles Tannock described Lambsdorff's comments as "nonsense", adding that, "Parliament's President Martin Schulz and sherpas Roberto Gualtieri and Elmar Brok from Socialists and EPP all agreed the UK deal."
He added, "Although many on the moderate Left may not like it, they know it's the price you pay you keep UK in the EU. My ECR group also supports the deal so I am confident it will have large majority in event of Remain vote."
He told the Parliament Magazine, "Any attempt to defeat it would cause a UK and EU constitutional crisis and an immediate campaign by the British government then recommending Brexit, which I would be forced to support due to breach of trust by an EU institution.
"On the Andrew Neill BBC politics show, even Ukip MEP Dianne James agreed it would go through in contradiction of Nigel Farage.
"Alexander Lambsdorff clearly wants the UK to leave by making such inaccurate and irresponsible comments during the referendum campaign. But it proves the point that Cameron gained real and substantial EU concessions from the renegotiations in spite of Eurosceptics claiming UK deal is worthless."
Further comment came from former UK Europe Minister, Denis MacShane, who said, "David Cameron's February deal did what was need at the time - to show that the UK, if it rejects Brexit, can expect some changes in the terms of its relationship with the EU.
"But to be put into EU law this requires treaty change, and I question whether any country has the appetite for a major new treaty which has to be submitted to referenda.
"Plenty of MEPs and other national ministers have said they are uncertain about aspects of the deal, so Lambsdorff is saying nothing new. But Cameron is the Prime Minister and unless MEPs want to see the EU break apart as other countries follow the Brexit example, it is better to urge UK voters to follow Cameron rather than castigate him."
Leading law firm Clifford Chance against Brexit
In other new developments, leading law firm Clifford Chance has come out against Britain leaving the EU in a letter to its staff.
Senior partner Malcolm Sweeting and managing partner Matthew Layton argue in the letter, "We are of the view that it is in the interests of the City, of many of our clients' businesses and of the firm for the UK to remain in the EU.
"The UK's relationship with the EU is also of crucial importance to many of our clients, not just in the UK but in Europe and around the world."
On Sunday, UK Labour MPs Ben Bradshaw, Emma Reynolds, Chris Leslie and Adrian Bailey urged Labour leader Jeremy Corbyn to "campaign relentlessly" in favour of staying in the EU.
They wrote in a newspaper article that, "Convincing the British public to remain in the EU should be the overriding priority for the political leadership this year.
"That's why we are calling on the leadership of the Labour party to campaign relentlessly for our EU membership with passion and without equivocation. This is overdue. It matters and it requires more than the occasional speech."
Meanwhile, an open letter signed by think tanks, migration campaigners and business groups, including Open Europe, Migration Watch, British Future and the Institute of Directors, calls on the UK government and both the Leave and Remain campaigns to clarify the consequences of Brexit on future immigration policy.
The letter states, "In our view, the public debate about immigration and free movement should be about future immigration policy - so that future changes should not apply retrospectively to those currently exercising their free movement rights."