The Cabinet minister in charge of leaving the European Union has defended the Government’s policy of doing absolutely no contingency planning for Brexit ahead of the referendum.

Oliver Letwin, who was appointed by David Cameron to lead the so-called “Brexit unit” the morning after the result, said it would not have been possible to do significant planning before the vote took place.

The aftermath of the Leave vote saw sterling fall to a three-decade low against the dollar and the FTSE fall sharply. Some major international companies have since announced plans to move their headquarters out of London.

Nearly two weeks after the vote there remains significantly uncertainty about what economic course Britain will take – with questions about freedom of movement, EU budget contributions, and single market access all in dispute.

Ministers were accused of a “dereliction of duty” by several members of the Foreign Affairs Select Committee after Mr Letwin tried to justify ministerial inaction in the run-up to the vote, despite polls showing a close race.

During the referendum campaign, Government spokespeople and ministers repeatedly confirmed that absolutely no contingency planning for Brexit was taking place.

A Downing Street spokesman told journalists at an official briefing the month before the referendum: “The Government has a position which is that we should vote to Remain and we are not contingency planning.” Asked to confirm that no planning whatsoever was taking place, he added: “That is the Government's position, yes.”

Mr Letwin, however, told the Foreign Affairs committee on Tuesday that extensive planning for the act of leaving the European Union would not have been possible.

“It was not possible to construct the negotiating strategy in advance, you couldn’t have imposed that on a new prime minister, and we are now going to be in the position, when the new prime minister comes in, to furnish that new prime minister with the grounds for carrying forward the strategy which that new prime minister and their cabinet will decide,” he said.

Oliver Letwin, the minister in charge of Brexit, gives evidence to the Foreign Affairs Committee (House of Commons)

When it was suggested by Crispin Blunt, the committee chairman, that Mr Cameron’s resignations had left him “holding the baby”, Mr Letwin replied: “I can only say that the baby is being firmly held, and that my intention is that the baby should prosper – because I care about the baby in question. It is, in fact, our country.”

Mr Blunt criticised the Government’s stance. “The current Prime Minister made it crystal clear in the run-up to the referendum that he was going to carry out the instructions of the referendum,” he said.

“I still put it to you that it was frankly a dereliction of duty that there was no contingency planning. There was never any suggestion that there would be a new Prime Minister after the EU referendum result.

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“Since there were only two options it might have been an idea that we would plan for both, don’t you think?”

Labour MP Yasmin Qureshi added: “There was a very great likelihood of us exitting and no one had come up with an exit plan to deal with one of the biggest challenges our country has faced in the last 50 years. That shows a clear dereliction of duty.”

Elsewhere in Parliament on Tuesday, legal experts warned that Britain would still likely be bound by international courts if it wanted access to the European single market.

Professor Michael Dougan, one of the UK’s foremost authorities on European law, advised the House of Commons Treasury Select Committee that membership of the European Economic Area would still see European law being given priority over British law.

“There is still an obligation for EEA states to respect the priority of EEA rules,” he told MPs in a fact-finding session.

“It’s not quite the same as the principle of supremacy of EU law for EU states, but it is still an obligation of priority for EEA rules to take preference over conflicting statutory rules within EEA states.”

He added: “In practice the scholarship from Norway and Iceland tell us there is not an enormous amount of difference from the effect of EEA law within those member states [and the effect of EU law in EU member states].”

Britain voted to leave the European Union by 52 per cent to 48 per cent on 23 June, prompting Mr Cameron to announce his resignation.