Euro MPs will not simply rubber stamp any deal on David Cameron's reform demands, the parliament's president has said, but he insisted he will work to secure any agreement reached in a crunch summit in Brussels this week.
The Prime Minister visited the Belgian capital for talks with European Parliament president Martin Schulz and other key players in an effort to keep his reform demands on track ahead of this week's meeting of EU leaders.
If a deal is reached by EU leaders at the summit on Thursday and Friday, MEPs will eventually have to approve parts of the reform package, including restrictions on benefits, but Downing Street has insisted any deal 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.
Mr Schulz said it was not possible for MEPs to simply accept anything put in front of them, but insisted that the parliament would be "constructive".
On the issue of welfare curbs, Mr Schulz said: "I gave the Prime Minister a clear signal for our commitment in the European Parliament to be as constructive as necessary and as possible, that we will try to find a fair compromise between the council, the commission and the parliament to try to find, in the way of concrete legislation, the necessary solutions for the problems described by the United Kingdom."
The parliament would begin the legislative process as soon as the UK voted to remain in the EU, he indicated.
He added: "I can't give a guarantee for the outcome of future legislation, nevertheless it is quite understandable that the Prime Minister asked the European Parliament to co-operate as intensively as possible, that was the assurance I gave to the Prime Minister, that we will do the utmost to find a fair deal."
Asked whether an agreement struck at the European Council summit on Thursday and Friday would be "legally binding" and the European Parliament would enact the necessary changes, 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 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."
Mr Schulz denied that MEPs effectively had a veto over any deal struck between Europe's leaders on Mr Cameron's demands.
Echoing European Council president Donald Tusk's assessment of the situation as "very fragile", Mr Schulz said: "Some political operations are always fragile, we have to find a compromise between 28 member states and then a compromise between the 28 member states and the commission and the parliament."
But he insisted that once a "frame" is agreed, possibly on Thursday, the European Parliament would get to work.
"This is not a veto, this is a normal procedure," he said.
"I encourage British people to vote on the basis of the outcome of Thursday, to vote Yes. Then we start a legislative procedure which will clarify and help to solve the problems addressed by the frame which will be, I hope, accepted on Thursday."
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."