Theresa May must act swiftly to manage down “sky high” public expectations about the financial benefits the UK might reap from Brexit, a senior Conservative MP has warned.

Andrew Tyrie said the Prime Minister needs to distance the Government from the “false prospectus” offered during the referendum by the Leave campaign, including its often-repeated claim that the UK could save £350 million a week, something which he said will prove “impossible to deliver”.

Mr Tyrie, who chairs the influential House of Commons Treasury Committee, said the Government should set out “an early, full and detailed explanation” of its negotiating position and an assessment of what it can realistically achieve, and should hold back on triggering talks until it has “clarity” from EU partners that they are in a position to offer reasonable terms.

If necessary, this could mean delaying the deployment of Article 50 - which kicks off the two-year exit process - until after French presidential elections in May 2017 or German polls in September next year, he said. And he insisted that MPs should be given a vote on Article 50 - something Ms May has said she will not grant them.

Mr Tyrie forecast that it will not be possible to achieve a new settled relationship with the EU within the two-year framework set out in Article 50, and called on the Government to agree transitional arrangements to avoid a “cliff edge” change in trading rules when the period runs out.

In a pamphlet for think-tank Open Europe, the Chichester MP said it was “clear that no-one knows yet” what Brexit will mean.

“If it is to secure consent for the terms of Brexit, and to restore public trust in political discourse, so damaged by the referendum campaign, the Government must be frank, both about the trade-offs involved, and the fact that many of the promises made by the Leave side are manifestly unfulfillable,” he warned.

Taking the right approach could result in “meaningful economic and political gains” from Brexit, but the wrong approach could wreak “early and possibly severe damage” on the country.

Mr Tyrie backed the Prime Minister's position that Britain should not accept an “off-the-shelf” model for its future relationship with the EU, based on models such as Norway, Switzerland or Canada, but needs its own unique arrangements suited to the UK's circumstances.

He said Britain should aim to negotiate “extensive access to the single market, some degree of influence over its rules, withdrawal from the customs union, and the restoration of control over free movement”, entrenched in a treaty with the remaining EU.

Top priority in negotiations should be securing continued access to the European single market for services, he said. And to avoid a “protectionist stitch-up”, the crucial financial services sector will need a deal which gives it not only access to the market but also influence over its rules.

The option of reverting to World Trade Organisation rules would “substantially curtail” UK exports and should be regarded as a “back-stop were negotiations to go disastrously awry”, said Mr Tyrie. Leaving the customs union will be essential to allow Britain to strike new trade deals with countries elsewhere in the world.

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On the issue of free movement, Mr Tyrie said the Government needs to make clear to voters that stronger controls “are likely to compromise the rights that UK nationals presently enjoy to live and work in the rest of the EU, and may also carry an economic and fiscal cost for the UK”.

And ministers face the difficult task of explaining to Parliament and the country that any deal may involve the UK continuing routinely to accept the rulings of the European Court of Justice, or losing much of its power to enforce its own trading rights. There should be “no bonfire” of EU regulations, which would inhibit trade and put UK companies at a commercial disadvantage, he said.

It was a “catastrophic misjudgment” for David Cameron to call the referendum without first securing robust proposals on how free movement and the status of EU law would be affected by British withdrawal, said Mr Tyrie.