Take a single difficult decision’

to attack Jeremy Corbyn as a ‘weak leader’ programmes would require a huge investment of effort from Whitehall at a time when officials will be tied up transferring thousands of pieces of European Union 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 three UK devolved executives.

The think-tank’s director Anand Menon, of King’s College London, said: “None of this is impossible, 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.”

He added that, far from reining in their ambitions, Theresa May’s Conservatives have offered “arguably the most statist and interventionist [manifesto] produced by a governing party in living memory”, while Jeremy Corbyn’s Labour Party proposes “a massive expansion of state control of the economy, direct and indirect, including the reversal of several of the major Thatcher-era privatisations”.

Prof Menon added: “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.”

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 also points out that the Office of Budget Responsibility has estimated the bill for reducing net migration to 185,000 will be about £6 billion a year.