David Cameron's campaign to rewrite the terms of Britain's membership of the European Union is wishful thinking likely to be rejected by many other EU member states, said a frontrunner to be the new head of the European commission in Brussels.

Martin Schulz, president of the European parliament, who is the centre-left's candidate to replace José Manuel Barroso as president of the commission in October, said he was open to listening to the prime minister's pitch, but was sceptical that Cameron would succeed.

"We need the UK in the European Union as a fully fledged member," Schulz told the Guardian in an interview. "But is it prepared to make the EU stronger and not weaker?"

The prime minister is sounding increasingly insistent that he will be able to secure a new EU deal for Britain by repatriating powers ceded to Brussels, through a renegotiation of EU treaties, before putting the proposed pact to an in-out UK referendum in 2017 if he is re-elected next year.

Speaking on Sunday about freedom of movement within the EU, Cameron said: "You've either got to change it with other European countries at the moment, or potentially change it through the treaty change that I'll be putting in place before the referendum we will hold on Britain's membership of the EU."

But there is little appetite in Brussels and other EU member countries for renegotiation and it is not clear if Cameron can deliver treaty change.

Schulz dismissed the attempt to cherry-pick the treaties to achieve bespoke membership terms tailored for individual countries.

"A new deal of permanent particular roads is wishful thinking," he said. "It is unacceptable for a lot of the other countries."

Rather than repatriating powers from Brussels, Schulz said he would be open to discussing "a re-delegation of duties" within the EU, arguing that the commission in Brussels was too big and tried to do too much.

This view coincides with that of Angela Merkel, the German chancellor. Schulz is a German Social Democrat whose party has just gone into coalition with Merkel's Christian Democrats in Berlin.

"I go with him [Cameron] as far as I share his views," said Schulz. "My message to the commission would be: what are the things we are doing here that we don't need to do here."

But he voiced mild frustration that Downing Street had yet to make clear what it wants from a proposed new dispensation. It is a view shared broadly across EU capitals.

"Cameron should make his suggestions for reform of the EU and then we can discuss it openly. I don't believe it is good to say 'these are my points and if you don't agree I'm going out'. It is not in the interests of the UK to go in this direction. Let's be pragmatic and not look at it ideologically."

Twice in the past fortnight, Cameron has threatened to veto future enlargement of the EU unless there are new curbs agreed on freedom of movement – a fundamental EU principle – for the acceding countries. In Brussels this is seen as playing to the nationalist anti-immigration gallery in Britain since there is scant prospect of any new members joining the EU in this decade.

Schulz warned that if Cameron were to lead Britain out of the EU following a 2017 referendum, he would be shooting himself in the foot by cutting the UK out of the huge transatlantic free trade zone currently being negotiated between Washington and Brussels.

"If the UK goes out, they will not be part of the transatlantic trade relationship. It's unthinkable," said Schulz.

But a British exit is becoming increasingly thinkable, with senior officials and diplomats in Brussels emphasising that the UK question will move sharply up the European agenda this year.

For the next few years, said one senior diplomat, "the existential questions in the EU are the governance of the eurozone, the treaty changes needed for that and whether the UK stays in and on what terms."

Cameron made it plain on Sunday that Britain now categorically rejected the notion of "ever-closer union" – another of the EU's fundamental principles.

Senior officials now say that it is entirely possible that there will be a "parting of the ways", dismissing talk of a two-speed or multi-speed Europe of individual countries heading in the same direction, if at different rates.

"It's not a slower train. The UK is off the train and not heading to the same destination. Or it's out of the EU," said a senior diplomat. "Both options are possible."