David Cameron’s campaign to ditch the EU’s mission as one of “ever closer union” has no chance of success, said the president of the European parliament, who also questioned the prime minister’s drive to bar other EU citizens working in Britain of claiming in-work benefits for four years.

On the eve of talks in Downing Street over breakfast with the prime minister, Martin Schulz said there was a “more or less unanimous” view among other EU leaders that the Lisbon treaty would not be reopened to accommodate Cameron’s requirements.

In his Guardian interview he said he was “surprised” by the British demand for an opt out from the EU’s historic commitment to create an ever closer union of the peoples of Europe. This would require a reopening of the treaty to amend wording that is seen in Europe as purely symbolic that has not stopped Britain from securing a range of optouts from EU integration over the years.

“To change it you need a treaty change,” Schulz said. “I see not so much chance for treaty change … Treaty changes take a ratification process in 42 chambers [across the EU]. This is a thing we should take into account. Treaty change debates will last years.”

“From time to time I’m a little bit surprised about this debate about ever closer union,” said Schulz. “It is not a threat for a country that is the UK. The ever closer union is not compulsory as Britain has shown in the past.”

I hope I return from London more wise Martin Schulz

He added: “I see no chance of changing the ever closer union, but I don’t even know if he wants to change it. I am really curious to learn directly from the prime minister what they want really … I hope I return from London more wise.”

Over breakfast with Cameron at Downing Street, Schulz will explore Britain’s EU renegotiation demands ahead of Cameron’s EU in/out referendum and a week before the prime minister tables his specific requirements to an EU summit in Brussels. Schulz is the latest in a line of EU leaders to rule out such negotiations, although Chancellor Angela Merkel of Germany is sitting on the fence on the issue.

Dara Murphy, the Irish foreign minister, told BBC Radio 4’s World At One: “The core focus at the moment is the strategy around negotiation to play a part in keeping the UK in the European Union. But, yes, it would be remiss of us given the possibility that our largest trading partner may be exiting the European Union, that is something we, of course, are looking at.”

Enda Kenny, Ireland’s taoiseach, will tell Cameron in talks in Downing Street that Ireland will be a firm supporter of continued UK membership of the EU amid fears that the Irish Republic would be severely damaged if its largest trading partner left the EU. But Kenny will make clear to Cameron when he visits London on Thursday that Dublin is wary of treaty change as he warns that such a move could trigger a referendum in the Irish Republic.

Schulz is also to see the UK foreign secretary, Phillip Hammond, and the leader of the Commons, Chris Grayling. It is the first time in over three years as parliament president that he has been officially invited to Britain in a signal that Downing Street is paying closer attention to the European parliament in the context of the referendum negotiations. Cameron also recently hosted Manfred Weber, a German christian democrat and the leader of the EP’s biggest caucus.

Schulz made clear that Cameron’s attempts to challenge free movement of labour in the EU would get nowhere, that the right to work anywhere in the EU was sacrosanct and “non-negotiable”. The government accepts this but wants to win changes to EU law to enable Britain to cancel tax credits and housing benefits for the first four years for low-paid EU citizens working in Britain. The governments of Poland and Romania have already voiced strong opposition.

The intervention by the German Social Democrat came as the Irish government confirmed that it is drawing up contingency plans for a British exit from the EU. A special unit, established in the Irish prime minister’s office in Dublin in recent months, will examine the impact of a British exit from the EU on the common travel area, in which people and goods circulate freely, that has been in place between both parts of Ireland and Great Britain since it became and independent state in the 1920s. Britain and Ireland joined the European Economic Community, a precursor to the EU, on the same date in 1973.

Frances O’Grady, the head of the UK Trades Union Congress, told the Guardian in Brussels on Wednesday that the TUC’s legal advice was that such changes to in-work benefits for non-Britons from the EU would also require treaty change.

“This is the thin end of the wedge, attacking everyone’s working conditions and using migrant workers as the battering ram,” she said.

Cameron wanted, she added, to use the negotiations to dilute the rights of working people across the UK, rights gained thanks to EU legislation such as those on maternity pay, equal pay, paid holidays, and the 48-hour maximum working week.

Schulz agreed that free movement of workers could not provide a license for so-called benefits tourism and conceded there was scope to negotiate on the issue without treaty change. However, he questioned the four years exemptions for low-paid workers sought by Cameron.

“We have to check if such a British law would be a breach of European rules,” he said. “If this is the case, he has to discuss with me and go into the reality … It’s justified to put the question on the table – are these tax credits only for UK citizens or are they for those people who work and pay and make contributions and are so low-paid? Is this compatible with European rules or not? A UK citizen in my country would get a tax credit because he is so low paid.”

The Schulz visit is part of the prime minister’s race to see all EU leaders on the referendum demands ahead of next week’s summit. Cameron received a boost on Wednesday when Matteo Renzi, the Italian prime minister, said that the EU without the UK is “impossible” to imagine. Speaking after a working lunch in Milan with Cameron, Renzi said: “For us it is a priority the UK can continue to work inside the European Union because a European Union without the UK is impossible.”

Cameron said that he and Renzi had “very good discussions on the importance of reform and change in Europe where I think we do have some common perspectives and some common ideas on the need for competitiveness, for flexibility.”

Schulz said he was going to London “to listen and learn” and to get a more concrete idea of what Cameron wanted. It was up to the prime minister to make that clear.

“The idea of the whole debate came from London. It was not born [in Brussels] or in another country,” he said. “David Cameron has now taken the initiative. He is now in the lead of the campaign in his own country and he should avoid becoming a hostage of some elements in his own party. He is now the negotiator in the UK and in Europe.”