THE EU chief Brexit negotiator has insisted the backstop requiring Northern Ireland to effectively remain in the European customs union and single market must be part of Britain’s withdrawal agreement if a deal is to be reached in October.

Speaking a day after the long awaited White Paper setting out the UK’s plan for its future relationship with the EU, Michel Barnier described the Irish border issue as “the most difficult” outstanding problem in the withdrawal negotiations.

“The most difficult is Ireland where we have to find a solution to avoid the return of a hard border,” he told the Carnegie Institute in Washington yesterday.

“I am very concerned about the situation...The situation in Ireland is not just about goods, controls, checks. It is about people, about peace.”

Barnier added that “we have to be very careful”, saying both sides had committed to protect the Belfast Agreement “in all its dimensions”, but said the EU could be flexible on the nature and substance of the backstop.

“We have proposed a backstop. It could be amended, challenged and it could be improved. But we need a backstop to ensure no border,” he said.

The issue of an Irish backstop got little mention in the 98-page White Paper published on Thursday with the UK Government maintaining it will not be needed as its future trade relationship with the EU will be based on free trade for goods and therefore a hard border would not be needed.

The document was unveiled yesterday amid shambolic scenes in the Commons after MPs did not receive a copy before the new Brexit Secretary Dominic Raab’s statement on it. It had already caused turmoil in the Cabinet during the week, prompting the resignation of Brexiteers, ex-Foreign Secretary and ex-Brexit Secretary David Davis, who rejected it for proposing a closer alignment between the UK and EU.

It sets out plans for an end to free movement, with new rules allowing visa-free travel for tourism and temporary work, and also calls for continued security co-operation, participation in Europol and a range of other EU agencies.

May is expected to face further difficulties over the paper in the Commons next week after angry Eurosceptics have submitted four amendments to the UK Government’s Trade Bill, arguing May has broken their trust.

Members of the hardline European Research Group (ERG) fronted by Jacob Rees-Mogg want MPs to kill off the Prime Minister’s customs arrangement proposals in an amendment to Monday’s bill, calling for the UK to refuse to collect duties for the EU unless member states do likewise.

A second amendment, which is backed by the DUP and Labour’s Kate Hoey, would force the government to agree in law to a commitment to never having a border in the Irish Sea. Hard Brexiteers have also pledged to mount a campaign of guerilla warfare against May’s government in an attempt to disrupt the White Paper’s proposals.