The government must set out in detail what it hopes to achieve from Brexit talks in order to restore public trust in politics, a senior Tory MP says.
Andrew Tyrie called for an “early, full and detailed explanation” of the government’s negotiating position.
The Treasury Select Committee chairman also said “sky-high” public expectations about the financial savings from Brexit had to be managed.
Mr Tyrie backed Remain in June’s poll which resulted in a Leave victory.
He urged ministers to “cast aside the damaging claim and counter-claim” of the referendum period.
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In a pamphlet written for the Open Europe think tank, he said the referendum’s “pernicious legacy” was to add to a “deep distrust in politics”.
“Politicians cannot afford to allow this to get any worse,” he said, saying the government had to be frank about the “trade-offs” involved with Brexit – “and the fact that many of the promises made by the Leave side are manifestly unfulfillable”.
He added: “Equally unfulfillable are the hopes of many Remainers, that the UK can carry on pretty much as now, and that a renegotiation can achieve continued membership through the back door.”
Parliament should get the opportunity to approve the UK’s negotiating position before Article 50 of the Lisbon Treaty – which begins a two-year exit process – is triggered, Mr Tyrie said.
Prime Minister Theresa May has rejected this option, however.
Mrs May has said she will not trigger Article 50 until the start of next year at the earliest, while some Brexit campaigners have called for it to be done immediately.
According to Mr Tyrie, the government should wait for “clarity” from other EU partners on what they will be able to offer before triggering formal talks, which could mean delaying Article 50 until German elections in September 2017.
Mr Tyrie, who based his article on evidence given to his Commons committee, said the negotiations over Brexit could secure “meaningful economic and political gains” but risked “early and possibly severe damage” if not approached properly.
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 EU.
Alongside access to the EU’s markets, Mr Tyrie also said “far-reaching” change was needed to the free movement of people. Curbs to migration have been the key demand of many Brexit campaigners since the Leave vote.
EU leaders have stressed that single market access is dependent on accepting free movement.
But Mr Tyrie said: “The idea that ultimate UK control over migration can be restored without fatally compromising the UK’s trade relationship with the EU is not unreasonable.
“Purism by EU negotiators on this point would not only be inconsistent with reality; it would also clash with other member states’ economic interests.”
The government needs to be clear that curbing immigration could carry an economic cost for the UK, “controversial though saying this remains”, he added.
Both the UK and the EU will “need to shed an instinct to grandstand” he said, warning that “exemplary punishment” for the UK over the Brexit vote could “backfire disastrously” on the rest of the EU.
In other Brexit news, a new campaign group is being launched to succeed the Britain Stronger in Europe group, which led the unsuccessful Remain campaign.
The cross-party Open Britain group said it would push for continued membership of the single market while making a positive case about the benefits of immigration.