Brexit talks will be on a "humongous scale" involving "difficult trade-offs", the UK's ex-EU ambassador says.
Sir Ivan Rogers predicted much of the talks would be "conducted very publicly" with "name-calling" and an "extremely feisty atmosphere".
He added that EU Commission chiefs were saying the UK should pay 40-60bn euros to leave and thought a trade deal could take until the mid-2020s to agree.
Sir Ivan, who had been due to leave his post in October, resigned last month.
In December he attracted criticism from some MPs when his warning to ministers that the European consensus was that a deal might not be done until the early to mid-2020s was revealed by the BBC.
Giving evidence to the Commons European Scrutiny Committee, he said this had come from a briefing he had written for Prime Minister Theresa May in October, based on the views of key figures in the EU and the remaining member states.
He said he did not know how it became public.
The government says it can conclude all separation negotiations, including a new free trade deal, within two years, having given notice of the UK's departure from the EU by the end of March.
Asked about the anticipated timescale, Sir Ivan said a comprehensive free trade agreement such as the one Mrs May was seeking would be the most comprehensive ever negotiated by the EU, and previous agreements have taken "an awful lot of time".
He said he believed an agreement with the UK could be concluded more quickly, but said the Brexit negotiation would be "unprecedentedly large" covering "huge tracts of Whitehall".
"It's a negotiation on the scale that we haven't experienced ever, certainly not since the Second World War."
Sir Ivan said there was a "big financial debate coming up" about the amount to which the UK should be expected to pay as it leaves the EU.
EU commission chief negotiator Michel Barnier and other key figures were "openly" saying the UK's total financial liabilities would be in the order of 40 to 60bn euros, Sir Ivan said, describing this as a "predictably hard line".
From the EU's point of view, UK withdrawal will "explode a bomb" under its seven-year budget, he said.
Sir Ivan told MPs the 27 remaining EU states would spend "an awful lot of time debating with each other" before negotiating with the UK, agreeing a common position.
Asked how confidential the negotiations would be, he said: "I think an awful lot will leak, Brussels is very leaky...stuff will get out, and incessantly in my view."
He added: "Expect an awful lot of this negotiation to be conducted very publicly."
Sir Ivan said that after a "phoney war period" talks "usually end up in a fairly mercantilist fist-fight" before finally resolving themselves in a deal of some sort.
A determination on both sides to make progress would be crucial, 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."
He said there was "no doubt" the UK would be able to negotiate free trade deals more quickly than the EU once it leaves, but said it may not have the same "negotiating heft".
Sir Ivan has been replaced by Sir Tim Barrow, a former UK ambassador to Russia.
Giving evidence to a separate Commons committee, International Trade Secretary Liam Fox said the suggestion the UK could face a bill upon leaving the EU was "absurd".
"I find it bizarre because the UK is using a legal power that we have under the Lisbon Treaty, a provision that was freely entered into by all our European partners," he said.
"Why should they then turn round to say that we should pay their costs for a process that everybody equally entered into at the time? So it seems to me an absurd argument."