Tony Blair's former chief of staff has savaged the UK Government's handling of the Brexit negotiations, accusing it of tabling "laughable, fantasyland" proposals.
Jonathan Powell said the process has been handled "pretty appallingly" and questioned whether any time was spent preparing in the year prior to negotiations with the EU starting in earnest.
Mr Powell, who is now involved in mediation work in conflict zones around the world, was relating his experience of negotiating during the Northern Ireland peace process to a headteachers' conference.
During his address to the Headmasters' and Headmistresses' Conference (HMC) he was asked to assess the performance of the UK's negotiators on Brexit.
He referred to the image of Brexit Secretary David Davis sitting at the negotiating table with EU counterparts in July with no documents in front of him.
"It was an extraordinary metaphor to see the British side, and particularly David Davis, turn up with precisely no pieces of paper in front of them and large piles in front of the EU negotiators," he told the audience of principals in Belfast.
"But actually a lot more problematic than that was then the fact it was the EU side that prepared all of the papers that were tabled before the summer break and the British Government didn't produce any.
"What is really important is framing a negotiation. When you start a negotiation you want to be the person setting the parameters."
Mr Powell, who was Mr Blair's top adviser throughout his time in Downing Street, added: "We have allowed the EU to set that negotiation and then, when we did produce papers during the course of the summer, frankly they are laughable, if you read them they are complete fantasyland.
"The ones for example that refer here to Northern Ireland are making proposals that are completely out of the question - there is no way they can work.
"It looks as if what they are doing is setting it up so they can denounce the foreigners for messing up the negotiation rather than negotiating seriously.
"That may be unfair but that's what it looks like if you just read the papers.
"Even though we had a year we seem to have not prepared for the negotiation.
"That's not the Civil Service's fault - they are extremely good and very effective at this stuff, though we actually lack any EU experience at the moment because all the EU players have all retired, so there is no-one there who has that experience.
"But the problem is more a political one because if you can't work out what you are going to achieve in a negotiation you are very unlikely to achieve it."
Mr Powell was delivering the opening address at the three-day HMC conference in Belfast.
The HMC is a professional association of heads of the world's leading independent schools.