Britain's former Ambassador to the EU Sir Ivan Rogers has warned that the UK’s Brexit talks with the bloc will descend into “name-calling” and a verbal “fist-fight”.

Sir Ivan, who quit his job after despairing at “muddled thinking” in Theresa May’s Government, said ministers should not underestimate the “humongous” task they now faced in negotiating the UK’s future relations with the EU

The former diplomat also said European officials simply did not believe the UK would quit the EU and revert to World Trade Organisation rules if it failed to secure a deal, a key threat that has been made by Ms May, and were likely to ask for between €40bn and €60bn (between £34bn and £41bn) as part of withdrawal talks.

Speaking to the European Scrutiny Committee, he then revealed that in the chaos after the referendum, Whitehall departments failed to maintain reports to his team as they struggled to cope with the prospect of Brexit.

He spoke to MPs just hours before the Commons debate and vote on triggering Article 50 launching the two-year period in which negotiations must be completed.

Sir Ivan said: “This is going to be on a humongous scale. We are going to have enormous amounts of business running up various different channels and then involve difficult trade-offs for Her Majesty’s Government and difficult trade-offs for the other 27 on the other side of the table.

Explaining that the UK and the EU 27 will come from “very different angles”, he said: “That involves generating a momentum and generating an atmosphere so that even when we get into name-calling and an extremely feisty atmosphere – and we undoubtedly will in both exit negotiations and future trade and economic negotiations – there is still an atmosphere to proceed and finalise agreement."

But he warned that talks “usually end up in a fairly mercantilist fist-fight” before finally resolving themselves in a deal of some sort.

Theresa May: UK left the EU to embrace the world

Sir Ivan quit his post, while urging his diplomatic colleagues in a letter to call out “muddled thinking” on Brexit and to “speak truth to power”.

He also highlighted that “serious multilateral negotiating experience is in short supply in Whitehall, and that is not the case in the [European] Commission or in the [European] Council”.

Sir Ivan told MPs on Wednesday that reports Brussels was preparing an “exit bill” of €40bn to €60bn for the UK were likely to be genuine.

The diplomat said senior EU figures believed the costs of crashing out without a deal and reverting to WTO rules were so great that Britain could not afford to refuse.

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He said: “The view of many will be that the implications for the UK of walking away without any deal on the economic side and without any preferential arrangement and walking into a World Trade Organisation-only world are ... so unpalatable that we won't do it."

He told members of the committee that reports routinely received from Whitehall departments, giving his team direction on what positions to take, dried up in the wake of the referendum, with the quality and quantity of documents suffering.

Sir Ivan’s position was thrust into the spotlight after advice he had given to Ms May was leaked to the press, ahead of a key European Council summit in December.