The president of the European Parliament has said there were no guarantees that the parliament would approve the new laws needed to copper fasten any deal negotiated by the UK on its future membership of the EU at a key summit in Brussels this week.
Martin Schulz said, however, that the Parliament would address any new legislation immediately after a British in-out referendum which is expected to take place in June.
He was speaking after a meeting with British Prime Minister David Cameron.
At the beginning of this month, European Council president Donald Tusk set out a 16-page draft agreement which is the basis upon which Mr Cameron will campaign for Britain to remain in the EU.
While the draft was immediately savaged by the right-wing press in Britain, its contents have since been carefully picked over by all the other member states.
Central and Eastern European countries are anxious about plans to limit in-work benefits to EU workers in Britain, and to index-link any children's allowance those workers send home to the standard of living in their own country.
France is also blocking anything that would allow Britain to veto by the back door eurozone integration, or that would give the City of London a commercial advantage in the realm of financial supervision.
While there are many technical issues that have to be resolved at the summit this week, the signals are that a deal can be done.
The Government is thought to be largely satisfied with the draft as it stands.
But the big question is, what will the British electorate think of it, and what if they reject the deal in a referendum which could take place as early as 23 June.
If Britain subsequently votes to stay in the EU, the European Parliament would still need to approve key elements of the deal.
"I can give you a guarantee that the European Parliament will, immediately after the referendum to stay in [Europe], legislate on the proposal of the Commission," Mr Schulz told reporters after his meeting with Mr Cameron this morning.
"But to be quite clear, no government can go to the Parliament and say: this is our proposal, can you give a guarantee about the result. This is not possible in a democracy."
Mr Schulz stressed that the role of the parliament was not to block a deal, responding to Nigel Farage of the anti-EU UK Independence Party, who said that the European Parliament would veto its terms.
An initial plan to meet the leaders of all the European Parliament's party blocs was cancelled - allowing Mr Cameron to avoid a confrontation with his eurosceptic domestic critic Mr Farage.
European Commission chief Jean-Claude Juncker has refused to entertain the idea of a British exit from the European Union, saying there is "no plan B".
"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," Mr Juncker said.
"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."
EU leaders will meet in two days to try to finalise a deal but there are signs that a number of member states have concerns about some of the details.
The measures, under negotiation, include curbs on welfare benefits offered to migrants.
Mr Cameron has said the proposed deal with the rest of the EU will be legally binding.
However, several parts of it would need legislative change that would need the support of the European Parliament and other EU countries after the British referendum on membership.
Britain, which does not use the euro, wants a deal that will protect it from ever having to pay into a fund to protect the eurozone.
It also wants any issues that affect all member states to be discussed by all 28 member states and not just the 19 nations that use the euro.
A French official said yesterday that more negotiations are required before a deal can be reached to avoid Britain leaving the bloc.