Theresa May's decision to trigger the countdown to Brexit opened up Britain to being "screwed" in the negotiations with Brussels, the UK's former ambassador to the EU has warned.
Sir Ivan Rogers, who resigned in January, said that invoking Article 50 without first establishing the sequencing of the talks had allowed the EU side to dictate the "rules of the game".
He said the remaining 27 member states were now refusing to move on to trade talks in order to "squeeze as hard as possible" over the UK's "divorce bill".
Giving evidence to the Commons Treasury Committee, Sir Ivan said he advised waiting until ministers were sure the negotiating timetable was going to work for the benefit of the UK before they triggered Article 50 last March.
However he said that he had been "heavily opposed" by "various people in London".
He said: "I did say last autumn I would not agree unequivocally to invoke Article 50 unless you know how Article 50 is going to work because the moment you invoke Article 50, the 27 dictate the rules of the game and they will set up the rules of the game in the way that most suits them.
"My advice as a European negotiator was that that was a moment of key leverage and if you wanted to avoid being screwed on the negotiations in terms of the sequencing you had to negotiate with the key European leaders and the key people at the top of the institutions and say 'I will invoke Article 50 but only under circumstances where I know exactly how it is going to operate.'"
Sir Ivan said that in the event, the EU side had done "exactly as you would expect" and insisted that they were not prepared to discuss the future partnership arrangements, including a free trade deal, until the terms of Britain's withdrawal had been agreed - including the financial settlement.
"If you were in their shoes that is exactly what you would do. You would think let's just maximise the pressure on the British side to move on money and squeeze as hard as possible because the debate they really want to have is about the future partnership," he said.
"So the more pressing that gets to them (the UK) and the more pressing it gets to their private sector to talk about transition, the more likely they are to be more generous with their money.
"Anybody could have told you that is exactly what the 27 would do."
Sir Ivan said Britain could not expect to enjoy broadly the same levels of access to EU markets after it had left, while ending free movement of people and the jurisdiction of the European Court of Justice.
"I don't think that's at all the mainstream European thinking," he said.
"The British can't simply expect the world to carry on broadly as is."