A wafer-thin majority could put the next PM in peril from day one

Politics Explained: The small margins between parties means that, whoever wins the Conservative crown, even a few dissenting MPs could have dire consequences for Britain’s new leader

In January 2017 Theresa May enjoyed a huge poll lead over Labour – the most commanding “political honeymoon” of any modern Conservative prime minister – as she marked six months in No 10.

It’s fair to say the new Tory leader – whether it is Boris Johnson or Jeremy Hunt – will see an abrupt end to their own honeymoon shortly after entering office, if they are indeed lucky enough to have one at all.

They will inherit the parliamentary arithmetic that plagued May’s government ever since the snap general election, resulting in three defeats to her Brexit agreement and the halting of any radical domestic reforms.

As it stands, the Conservatives have a working majority of just four, immediately placing any substantial vote on a knife’s edge in the House of Commons. This will not change with a new prime minister, despite their visions of breaking the impasse in British politics.

Critically, the working majority could be reduced further in the coming weeks, if Chris Davies fails to win back his seat for the Tories after being dumped by his constituents on Friday.

Following a conviction for faking expenses claims, more than 10,000 of Davies’ constituents demanded a by-election in Brecon and Radnorshire, and now he has to fight off a resurgent Liberal Democrats.

There is some speculation Labour could force a vote of no confidence in the first week of a Boris Johnson government – a major test of whether it could hold support in the Commons.

But it is the autumn months that are potentially fatal to a Johnson administration, faced with the looming 31 October Brexit deadline and his commitment to take the UK out of the EU by this date come what may.

If he fails to reach an agreement – one that is accepted by the House of Commons – he could push to leave the bloc without a deal, and almost certainly trigger a no-confidence vote, a last-ditch attempt by the opposition parties at Westminster to stop a no-deal scenario.

Two senior Tories – Ken Clarke and Dominic Grieve – have already gone public, saying in recent weeks that if a prime minister were to push through Brexit without a deal then they would vote against the government in a confidence motion as a last resort.

And cabinet minister Amber Rudd believes there will be a sufficient number of Conservative rebels to vote down the government and risk a general election.

Given that a general election is the only route to substantially changing the parliamentary arithmetic, Johnson himself may feel like calling one before this “nuclear option” is triggered by MPs in the Commons.