'No Plan B,' Says Juncker as Cameron Holds Crunch Brexit Talks

European Commission chief Jean-Claude Juncker said Tuesday he could not imagine Britain leaving the EU as Prime Minister David Cameron drummed up support in the European Parliament for his reform proposals.

Cameron came to Brussels for talks with Juncker, European Parliament head Martin Schulz and top MEPs aiming to win them over to the changes he says Britain must get to avoid “Brexit”.

Schulz, a dyed-in-the-wool supporter of the whole European Union project, pledged that parliament would be an "honest partner" for Britain but it also had "concerns to raise."

"The European Parliament will move very quickly on the proposals ... but I cannot make any guarantees on the outcome," Schulz said.

"The European Parliament will do its utmost to support compromise and a fair deal."

Parliament will have an important say on any agreement, but Cameron must first strike a deal at a summit of all 28 EU leaders in Brussels on Thursday and Friday.

If Cameron gets what he wants there, he will take the deal into a proposed referendum, most likely in June. If he wins the vote, then the deal returns to Brussels to be put in legal form.

Parliament will be closely involved in that process alongside the Commission, the EU's executive arm, and the bloc's 28 leaders.

A source in the European People's Party, the biggest group in parliament, firmly dismissed accusations by British euroskeptics that MEPs would unravel the deal.

"If we have a 'yes' from the British voters, then we will deliver," he said.

The discussions in Brussels are part of a frenetic merry-go-round of diplomacy ahead of this week's summit.

Juncker, speaking before meeting Cameron, said he refused to even entertain the idea of Britain leaving the bloc.

"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," 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. Britain will stay in the European Union as a constructive and active member of the Union."

Cameron made no public comment during his four-hour visit to Brussels.

EU chief Donald Tusk meanwhile repeated his warning that negotiations were at a "fragile" stage and nothing could be taken for granted.

"The proposal I have put on the table is a fair and balanced one," Tusk said during a stopover in Athens on a tour to prepare the ground for the summit.

"It helps the UK to address all the concerns raised by Prime Minister Cameron, without compromising on our common freedoms and values," he said.

"There are still many difficult issues to solve."

Cameron's most controversial proposal is to restrict welfare payments for four years for EU citizens working in Britain.

Many of the EU's eastern member states such as Poland, which has hundreds of thousands of its workers in Britain, say such a measure would discriminate against them and undercut the core bloc principle of freedom of movement.

Initially, Cameron's demand that non-euro countries such as Britain have safeguards against closer integration of the single currency area was expected to be relatively straightforward but it has now run into serious opposition, especially from France which says London must not have a veto over the eurozone.

An opt-out from the EU's mission of "ever closer union" and strengthened national sovereignty are also facing unexpectedly strong opposition.

The prime minister agreed to hold the referendum largely to head off gains by the anti-EU UK Independence Party which was exploiting sharp differences over Europe within his own Conservative Party.

Cameron met the heads of the two largest groups in parliament to deliver his message that his reforms are in the bloc's best interests.

But he came under fire after cutting plans to meet all of the parliament's political groups, with UKIP leader Nigel Farage accusing the prime minister of being a "chicken" for pulling out.