Brexit will be a "humongous" task as the negotiations between Brussels and the UK may descend into a "fist-fight", Sir Ivan Rogers said on Wednesday (1 February). The former top British diplomat to the EU issued the warning to the House of Commons' European Scrutiny Committee.

"This is a negotiation on a scale we haven't experienced since the Second World War, it's going to be on a humongous scale," he told the group of MPs.

Rogers, who quit as the UK's permanent representative to the EU in January, explained the parties were in the "phony war" period as Theresa May plans to invoke Article 50 and trigger talks on 9 March.

"Trade negotiations always start with pious and pro-free trade comments on both sides of the table, they usually end with a mercantilist fist-fight," he said.

"We are in the phony war period at the moment on both sides of the [English] Channel – the negotiations haven't started, the players aren't on the pitch.

"When they are on the pitch, the 27 will have to organise themselves and they are in the process of organising themselves."

Rogers also warned that the talks, which are expected to last two years, will be "immensely complex" as the 27 other EU nations attempt to co-ordinate with chief European Commission negotiator Michel Barnier.

"You've seen already at leader level they are organising themselves in order to be able to negotiate with us and they'll have a huge internal negotiation to go through," he said. "This is not widely enough understood in this country that the 27 will spend an awful lot of time debating with each other and producing agreed lines and agreed mandates for their own negotiator."

Rogers, elsewhere, confirmed that the EU Commission was considering a divorce bill of up to €60bn for the UK, based on liabilities.

He also stressed to the MPs that he would "never leak" after a memo of his warning that the Brexit negotiations could take up to 10 years was made public. "I never leak, I never have under any government, I never would," he protested.

Rogers later quit the civil service and was succeeded by fellow career civil servant Sir Tim Barrow, the UK's former ambassador to Moscow.

His evidence to the European Scrutiny Committee comes as MPs continue to debate the government's Article 50 bill. A vote on the second reading of the draft legislation in the Commons is expected around 19:00 GMT.