Sir Ivan Rogers, the former EU ambassador, is appearing before MPs on Commons European scrutiny committee.

He says that EU negotiations will be on 'humongous' scale not experienced since the second world war.

Sir Ivan warns that the egotiations to leave the European Union are likely to descend into "name-calling" and "fist-fighting" before any agreement can be found

"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", he says.

"Obviously, I would expect us and the 27 to be coming at this from very different angles and with very different objectives, but negotiations ultimately only culminate in deals if there is a determination on both sides of the table to make progress.

"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."

Sir Ivan says that member states negotiating positions will leak as Brussels is a "very leaky place".