David Cameron has been warned that Euro MPs will not simply rubber stamp his reform demands if he secures an agreement at a crunch summit this week.
The Prime Minister was told there was no guarantee that measures would make it through the European Parliament unchanged, with one MEP warning it was a "distinct possibility" that the plans could face a "very hostile" reception.
Mr Cameron held a series of meetings with key players in Brussels in an effort to keep his proposed deal on track ahead of the gathering of European Union leaders on Thursday and Friday.
If a deal is reached, MEPs will eventually have to approve parts of the reform package, including restrictions on benefits, but Downing Street has insisted any agreement would be a "legally binding document under international law, entered into by the 28 leaders of member states" and that the European Parliament should deliver on that.
The parliament would begin the legislative process as soon as the UK voted to remain in the EU, the parliament's president Martin Schulz indicated.
Mr Schulz said that once a deal is struck "there will be a very constructive debate" among MEPs.
"But to be quite clear: No government can go to a parliament and say: 'This is our proposal, can you give a guarantee about the result?' This is, in a democracy, not possible.
"Therefore my answer is the European Parliament will do the utmost to support compromise and a fair deal, but I can't pre-empt the result in the European Parliament.
"But, once more, once the institutions agree, our experience is it goes in a good direction."
Hungarian MEP Gyorgy Schopflin warned that the European Parliament could cause problems for the deal.
Asked if MEPs could "wreak havoc", he told BBC Radio 4's World At One: "I think that's a distinct possibility. That depends on the shape and concept of the deal - we won't know this until the early hours of Friday morning.
"But I think the European Parliament will obviously look at it very closely and there will be some groups - I suspect the Greens - who will be very hostile to it indeed, unless it is pointing towards more integration, which I don't see happening."
Ukip leader and MEP Nigel Farage said: "There are many groups here who are spoiling for a fight.
"The real truth is that this deal is not worth the paper it's written on. It is subject to European Parliamentary approval and ultimately judgments of the European Court of Justice."
Britain's renegotiation is the first item on the agenda for the European Council summit, but the gathering of 28 EU leaders is not scheduled to conclude until Friday lunchtime, after which Mr Cameron will call an immediate Cabinet meeting if he secures a deal.
The meeting will effectively fire the starting gun on the referendum battle, as Eurosceptic ministers will be given the green light to campaign for a Leave vote in the poll expected on June 23.
In a sign of the unease felt in parts of Europe about the proposals to curb welfare payments, Czech minister for Europe Tomas Prouza said the measures will only apply to newcomers rather than existing claimants, and he suggested that other EU countries should not be able to follow the UK's lead.
He told the BBC Radio 4 Today programme: "In central Europe there has been willingness to help the UK and there still is, but the issue we have is not with the UK and David Cameron's demands, the issue is with other countries trying to piggyback on the British proposals for their own benefit."
He added: "The proposals are clear that the limits on in-work benefits would apply only to the newcomers as it's a very UK-specific solution, so we need the very same guarantees also for the child benefits indexation that applies only to the newcomers and only those working in the UK.
"It's important we don't do the changes retrospectively."
European Commission president Jean-Claude Juncker, who also held talks with the Prime Minister in Brussels, said there was no "plan B" in preparation for a possible Brexit because the UK would remain a "constructive and active member".
He said: "If I would say now that we have a plan B, this would indicate a kind of willingness of the Commission to envisage seriously that Britain could leave the European Union.
"So I am not entering into the details of a plan B, because we don't have a plan B, we have a plan A. Britain will stay in the European Union as a constructive and active member of the Union."