UK prime minister Boris Johnson’s defenestration by the Conservative party will cast a long shadow on British politics whichever candidate succeeds him.

Brexit is unfinished business, the economy is fragile and the UK’s relations with Europe have yet to stabilise.

Relations with Ireland are also due a reset, and could hardly be worse.

The Tory party has led the UK government since 2010. With the next election not due until 2024 and a strong majority in the House of Commons, the next leader should see out a 14-year stretch of Conservative government under four prime ministers.

The first three have led the country up a series of blind alleys. There are several candidates capable of repairing the damage but no certainty that a new course will be found.

David Cameron will come to be seen as the most consequential of the three, since the 2016 referendum and the strategic error of Brexit was entirely Cameron’s handiwork.

He inherited the fallout from the financial crisis and his handling of economic management was criticised. But he will be remembered for the decision to call a referendum on Britain’s EU membership in fear of a fairly small Eurosceptic tendency in the Conservative party and at a time when Europe barely registered as a voter preoccupation in the opinion polls.

The UK does not have a written constitution and there had rarely been national referendums on any issue. Their constitutional status is simply undefined.

The 2016 expedition to the ballot box was a war-of-choice rather than something unavoidable, and Cameron’s motivations were expedient. He decided in 2013 to spin the bottle on EU membership for reasons to do with Tory party management and alarm that Nigel Farage’s UKIP party had been siphoning away votes from the Tories on a Europhobia ticket.

Cameron expected to win what he should have known was a 50-50 contest, lost narrowly and headed for the hills the following morning, leaving the mess to Theresa May.

She failed to clear it up, in large degree because Cameron left no plan to execute Brexit should Leave win the vote, an extraordinary omission. Remember the agonised debates about a soft versus a hard Brexit, how to interpret “the will of the people”, the demands for a second referendum as the full implications began to emerge?

Her three years in office were devoted to an agonising attempt to fashion an EU exit strategy out of the meaningless commitment that “Brexit means Brexit” after the electorate had given its verdict blindfolded.

Whoever wins the Tory leadership will have the opportunity, and the time, to fashion a healthier economic and political relationship with Britain’s neighbours

May was unable to fashion a coherent policy and was ejected in 2019 for Boris Johnson who won the December election on an empty, three-word slogan, “Get Brexit Done”.

The blustering fatuity of Johnson contrasts with the measured irresponsibility of Cameron, who put an unadorned Leave or Remain choice to the electorate without any consideration of the UK’s withdrawal strategy.

There was not even a Royal Commission before the vote to tease out the options and inform the public, many of whom seemed to think they were voting to limit non-EU immigration or to relieve the UK of financial contributions to Brussels, dishonestly exaggerated by Leave campaigners.

Whoever wins the Tory leadership will have the opportunity, and the time, to fashion a healthier economic and political relationship with Britain’s neighbours.

All that has been achieved under Johnson is a bare-bones trade agreement which avoids tariffs but sees the UK outside both the EU’s single market and outside the customs union. This means onerous border checks and non-tariff barriers arising from divergence in regulations for merchandise trade.

None of this was required in order to comply with the referendum decision to quit the political structures of the European Union, but the most abrupt and damaging form of Brexit was chosen in the subsequent welter of Europhobia and there is no going back.

The opposition Labour party has now ruled out any closer relationship with either the single market or the customs union and it is impossible to see a future Tory leader doing so.

There will be lasting economic damage even if the current difficulties about the Northern Ireland Protocol can be resolved

Britain is likely to be a third country, with no greater access to the European market than Brazil or India, for the foreseeable future and a change of government in 2024 will not make any difference. This is a very bad outcome for Ireland, which had closer economic relations with the UK, to mutual benefit, before the two countries joined the EU in 1973.

There will be lasting economic damage even if the current difficulties about the Northern Ireland Protocol can be resolved.

This is the true harvest of the 2016 referendum, and it is no consolation that the British electorate never voted for this outcome since they were never asked.

The Conservative party under its last three prime ministers has lost its claim to the safe-pair-of-hands trademark.