Lord Gus O’Donnell, who served as the cabinet secretary between 2005 and 2011, said Britain would be forced to give something in return in order secure the deals it wants with the European Union.
Speaking to Radio 4’s Today programme, the economist, an outspoken critic of the EU divorce, said the job on the country’s hands was “massive”.
He 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.
“We are in a plane being flown by members of the EU and we’re about to jump out and we’ve got a parachute that was designed by the people flying the plane.
Lord O'Donnell said the UK had a "massive" job on its hands
We are in a plane being flown by members of the EU and we’re about to jump out and we’ve got a parachute that was designed by the people flying the plane
Lord Gus O'Donnell
“And they’ve designed it in a way to deter anybody else jumping out.”
Lord O’Donnell said the European Council’s “big beasts” would not be easy to negotiate with on “tricky issues”.
He said: “So we’re going to start off saying we don’t owe you anything and they will say you’ve been with us for a long time 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, the free movement of labour.
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Theresa May signing the Article 50 paper
“So there’ll be a set of issues at the end and it will be a big political deal.”
The comments come as Article 50 is set to be triggered on Wednesday, kicking off a two-year negotiation period.
Theresa May is expected to promise in a statement to MPs: "When I sit around the negotiating table in the months ahead, I will represent every person in the whole United Kingdom – young and old, rich and poor, city, town, country and all the villages and hamlets in between, yes, those EU nationals who have made this country their home.
"It is my fierce determination to get the right deal for every single person in this country.
"For as we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can – and must – bring us together.”