Chief EU negotiator Michel Barnier has branded Theresa May’s call to hold simultaneous withdrawal talks and trade negotiations “risky”, as he underlined his determination to settle the UK’s Brexit divorce terms first.

In a speech, Mr Barnier said it was necessary to cement conditions of Britain’s withdrawal before discussing future relations in order to “build trust”.

He also emphasised the need for early talks to include a “single financial settlement” to cover the costs of Britain’s departure from the EU and its outstanding liabilities.

He was speaking as the European Parliament formally approved its negotiating guidelines, which will feed into the final mandate he is given to take into talks with the UK’s Brexit Secretary David Davis.

After Ms May repeatedly called for withdrawal and trade talks to take place at the same time in her letter triggering Article 50, Mr Barnier said: “This is a very risky approach. To succeed, we need on the contrary to devote the first phase of negotiations exclusively to reaching an agreement on the principles of the exit.

“We are not proposing this to be tactical or to create difficulties for the UK. On the contrary, it is an essential condition to maximise our chances to reach an agreement within two years.

“This is also our best chance to build trust before proceeding to the second phase of negotiations. This second phase will be devoted to scoping our future relations and discussing necessary transitory arrangements.”

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He added: “The sooner we agree on the principles of an orderly withdrawal, the sooner we can prepare our future relationship in trade, but also in security and defence.”

Ms May repeatedly tried to tie security to trade talks in her Article 50 letter, leading to a furore that only ended when Downing Street called European leaders to assure them it was not a threat to withdraw security cooperation.

The Prime Minister is also under pressure from the right of her party to minimise the ‘divorce bill’ payment handed to the European Union on leaving, the size of which the bloc is insisting will form part of early withdrawal talks.

Mr Barnier said: “There must be a single financial settlement, covering all commitments made by the United Kingdom as a Member State.

“Here again, the Resolution is unambiguous. It is very important.