The report by a group of independent experts based at King’s College London said the Brexit deal will affect everything from trade to foreign policy and its impact has been underplayed by all the major party manifestos.
The Conservative, Labour and Liberal Democrat manifestos all operate on the assumption that – aside from the negotiations themselves – it will be “business as usual” for government, said the report by the think tank UK in a Changing Europe.
Ambitious plans for state intervention included in both Tory and Labour programmes would require a huge investment of effort from Whitehall at a time when officials will be tied up transferring thousands of pieces of EU law onto the British statute book, developing new regulatory structures to replace European rules and sorting out new divisions of power between Westminster and the devolved executives.
“None of this is impossible,” said the think tank’s director Anand Menon, of King’s College London.
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“But it will require careful planning and the commitment of considerable resources.
“And it is frankly hard to see how much else can be accomplished whilst all this is being done.
“Can we really expect ambitious plans for schools, or the NHS, or anything else for that matter to be rolled out during the next few years? I certainly do not.
“But none of the manifestos takes such diminished capacity into account.”
Far from reining in their ambitions, Theresa May’s Tories have offered “arguably the most statist and interventionist (manifesto) produced by a governing party in living memory”, while Jeremy Corbyn’s Labour proposes “a massive expansion of state control of the economy, direct and indirect, including the reversal of several of the major Thatcher-era privatisations”, he said.
“Immediately after the referendum, much criticism was levelled at the government of David Cameron for failing to consider the implications of a vote to leave the European Union,” said Prof Menon.
“Almost a year on, much the same could be said of the party manifestos ahead of the General Election.”
He added: “What a shame that the parties haven’t properly factored it in to their plans.”
The report – entitled Red, Yellow and Blue Brexit – notes that “almost all” economists predict short-term economic costs from Brexit as the UK adjusts, with recent research suggesting that falling back on World Trade Organisation rules would reduce annual GDP by around 3 per cent.
And it points out that the Office of Budget Responsibility has estimated the bill for reducing net migration to 185,000 at about £6 billion a year, and suggests that hitting the 100,000 target set by Tories may cost a similar sum.
Labour’s hopes of maintaining the benefits of the single market while ending free movement are “fatally flawed”, while its rejection of withdrawal without a deal may be undeliverable because of the Article 50 deadline under which the UK must leave on March 29 2019, whether or not agreement is reached, said the report.
And Lib Dems, who promise a second referendum on any deal, would be “caught between negotiating a very close relationship with the EU and arguing such a relationship would not be preferable to remaining”.
Prof Menon said: “What is striking, is that while all three parties view Brexit as a major event, the manifestos treat it largely in isolation from other aspects of policy, rather than the defining issue of the next parliament.”
Additional reporting by Press Association.