Manfred Weber says divorce bill is likely to be easier to resolve than issues of citizens’ rights and Irish border

A middle way should “easily” be negotiated between Britain’s claims to have no divorce bill to settle when it leaves the EU and Brussels’ position that the country could owe as much as €100bn, the leader of the largest group in the European parliament has said.

In the first significant sign of a willingness on the EU side to compromise on the highly contentious issue, Manfred Weber, chair of the centre-right European People’s party, of which Angela Merkel is a leading light, suggested the bloc could be open to reducing its initial demands.

Weber said the UK’s need to fulfil its commitments in the EU budget up to the point it leaves in 2019 formed a “solid basis” from which to start the talks. But he appeared to suggest Theresa May might be able to reduce Britain’s total bill by pointing to its share of the bloc’s £130bn assets, which would include buildings, investments and even its generous wine cellar.

With reference to the opening negotiating issues of citizens’ rights, the divorce bill and the border on the island of Ireland, Weber told reporters: “Frankly speaking, I think the bill question will be finally, probably, having the three priorities in mind, will be the easiest one to solve. In the end, the British position is clear. Our position is clear. And then we have to find a middle way.”

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The position of the German MEP, who is close to Merkel, appears to clash with that of the European commission, which is insisting the UK has no claim to ownership of EU assets.

EU officials have said the UK’s liabilities in the so-called Multi-annual Financial Framework (MFF), such as the need to pay officials pensions, and fund currently budgeted projects, cannot be offset by its share of the assets, because it has no ownership rights under the treaties. The EU’s chief negotiator, Michel Barner, has said the bill when it is presented should be “incontestable”.

When future payments into the common agricultural policy are factored in, it is claimed Britain could be left with a bill of €100bn (£84.5bn), a figure described as “the rough and tumble” of negotiations by the UK’s Brexit secretary David Davis. May is said to have suggested to Jean-Claude Juncker, the European commission president, over dinner at Downing Street that the UK does not legally need to offer a penny as it leaves.

Weber told reporters in Strasbourg: “The MFF is a very solid base where we can calculate what are the demands, the payments, necessary to work on the current MFF, and Britain will until the end benefit from the MFF so that is a very easy task, I would say. And the rest is up to negotiations. That is not the most problematic part.”

Weber added that the issue of avoiding a breakdown in peace in Ireland would be the hardest issue to deal with.

He made his comments after being asked to respond to an attack by a Belgian MEP, Phillippe Lambert, the leader of the Green group in the European parliament, on the EU’s current stance on Brexit talks.

Lamberts said at a press conference that both sides of the negotiations seemed to have become radicalised in recent weeks, but that the EU should maintain its “credibility”. “If we say the UK has to share its part of the liabilities, we can’t at the same time say the UK has no claim on any of assets,” Lamberts said. “That can’t be said. If they have the one, they have the other.

“Obviously if the UK has to shoulder its entire state of liabilities it must enjoy the benefits of its share of the assets and that will of course be offset against the liabilities to be paid. Its a straightforward accounting principle and I think the Europeans are making themselves a laughing stock by failing to recognise that. We have to have a decent, calm atmosphere for the negotiations. We have to keep the advantage of credibility on our side.”

The European commission was pushed into its tough line on EU assets by member states, the Guardian understands. The latest leaked negotiating directives do not appear to have been weakened in recent weeks after consultation with member states.

The document states: “The agreement should contain a calculation of all obligations that the United Kingdom has to honour in order to settle its financial obligations towards the union budget, all institutions or bodies established by the treaties, and other issues with a financial impact.”

The directives now specifically reference the need in time for certainty for financial services in the City of London, in what will be seen in Downing Street as a promising softening of the EU’s position. The document now says: “Matters, such as services, where there may be a need to reduce uncertainty or avoid a legal vacuum will be covered by subsequent negotiating directives.”

Previously the French, who hope to win over some of the City to Paris, had been keen for the issue of financial services not to be mentioned at all in the EU’s negotiating directives, and mention had been omitted.