Cameron was in the Belgian capital for discussions with the parliament’s president Martin Schulz and senior MEPs who could play a major role in scrutinising any deal on a new settlement for the UK.
The meetings in Brussels follow talks in Paris with French president Francois Hollande, where the two leaders agreed that the draft proposals for reforming Britain’s membership of the EU provide a “firm basis” to agree a deal this week, according to Downing Street.
Downing Street’s assertion suggests French concerns over protections for non-eurozone states and special treatment for the City of London have been calmed.
A Number 10 spokesman said: “They agreed that we are making good progress on the UK renegotiation and that the draft text from the European Council provides a firm basis to reach agreement at this week’s summit.”
In Brussels, Cameron was also holding talks with leaders of the centre-right EPP and the Socialist grouping, and three MEPs who have been acting as “sherpas” in the negotiations.
MEPs will eventually have to approve parts of the reform package including restrictions on EU citizens’ 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.
But the Vote Leave campaign has claimed any deal will have the “legal weight of an unsigned contract” because none of it will be enshrined in EU treaty change.
The group claimed the European Court of Justice will only accept a ratified change of EU treaties as legally binding and said the court had overriden an agreement made between Denmark and and the EU in 1992 and ignored UN Security Council resolutions.
Syed Kamall, leader of the European Conservatives and Reformists group in the European Parliament, told BBC Radio 4’s Today programme that Mr Cameron was determined that MEPs should back a deal.
The Tory MEP said: “We want to make sure that the deal the British people vote on is actually the final deal and that the European Parliament doesn’t make any changes.
“That’s one of the reasons that David Cameron is in Brussels today. He’s talking to the big leaders of the parliamentary groups… to try and make sure he has support to get these changes through parliament.”
On Monday, European Council president Donald Tusk warned that Mr Cameron’s renegotiation was “very fragile” and unless handled carefully could lead to the break-up of the union.
Mr Tusk is undertaking a whirlwind tour of EU capitals including Berlin, Paris and Athens to sell the package of reforms he drafted in response to Mr Cameron’s demand for change.
Britain’s renegotiation is the first item on the agenda for the two-day European Council summit in Brussels beginning on Thursday.
However, 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 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.
Mr Prouza, the Czech Europe minister, 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.
“It’s in the proposals as we understand, and Donald Tusk when he comes to Prague later today should be able to confirm it.”