The scale of the Brexit challenge facing the Government is greater than going to war, a former head of the civil service has claimed.
Lord O'Donnell said leaving the European Union was like leaping from a plane with a parachute "designed in a way to deter anybody else jumping out".
Meanwhile, former senior Brussels official Jonathan Faull warned it would be difficult to complete a trade deal within the two-year timetable set out by Article 50.
Setting out the task facing officials and ministers, former mandarin Lord O'Donnell said: "This is much bigger than anything I did - coalition, wars - this affects all parts and all the people in the country, so it's massive."
He added: "We are in a plane being flown by members of the EU and we are about to jump out and we have got a parachute that was designed by the people flying the plane, and they've designed it in a way to deter anybody else jumping out."
Lord O'Donnell told BBC Radio 4's Today programme the major issues - including demands for the UK to pay a fee to leave the EU - would be decided by the "big beasts" of Europe, the leaders of the individual member states.
He said: "We are going to start off saying 'we don't owe you anything' and they would say 'you've been with us a long time and there are these ongoing commitments'.
"In the end the European Council, the big beasts, will be around the table and one of the things that will be outstanding will be money, the other one will be really tricky issues like agriculture, there will be free movement of labour - so there will be a set of issues at the end and it will be a big political deal."
Mr Faull, who led the European Commission's Brexit taskforce ahead of the 2016 referendum, told Today that it should be possible to reach an agreement on the UK's withdrawal from the EU within the two-year timetable, but talks on trade may not fit within the deadline.
"I'm sure there will be some consideration of that once the early work on the terms of withdrawal has reached the appropriate stage," he said.
"Whether within two years it is possible to deal with every last sector of industry, every last rule and arrangements for what happens afterwards, I think is going to be difficult.
"We know that trade agreements of that sort take a very long time and this is going to be a very big trade agreement."
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