To stay is overwhelming be fighting for his job

immediately after the result. If he loses this vote, Mr Cameron’s career as Prime Minister will be over. The Tories will then choose a new leader in an election in which he will be unable to stand.

Yesterday, Mr Cameron insisted again that he would not not resign, whatever the outcome. “It’s very important that the individual careers of individual politicians don’t get caught up in this question,” he said in an interview.

To be sure of being safe, Mr Cameron must win by an overwhelming margin, such as 60 per cent for Remain to 40 per cent for Leave.

But with polls pointing towards a close result, Downing Street has already begun working on a plan to save his leadership. He is likely to try to seize the political agenda with a policy blitz, focusing on social mobility, with a new “life chances” strategy.

Key decisions will also be taken on Heathrow airport, with possible votes on a replacement for Trident.

In a statement outside No 10 after the result is declared, Mr Cameron may offer new concessions, with fresh plans for tougher action on migration in an attempt to appease his Brexit-supporting colleagues and reunite the party and the country.

If Remain win by a tiny margin, Mr Cameron could also be forced to carry out an immediate “rescue reshuffle” in an attempt to reassert his authority. If Remain wins emphatically, the PM will have more time and may delay his reshuffle until the autumn.

Either way, leading Brexit campaigners will need to be nurtured and promoted within government to heal the deep divisions within the Tory party.

Boris Johnson is expected to be offered a role in a mid-level Cabinet job, possibly as transport secretary. This would put the former mayor of London in a difficult position, since he would be forced potentially to oversee the expansion of Heathrow airport, which he has promised vigorously to oppose.

Figures close to Downing Street have hinted that Liam Fox, the pro-Brexit former defence secretary will return to the Cabinet. However, pro-Brexit Tories are warning that MPs will be “on strike” whatever the result, and will refuse to back Mr Cameron and George Osborne in key Commons votes.

If the Prime Minister cannot get any of his reforms past his own MPs in the Commons, government will become impossible and another election may be needed.

Many economists expect the markets will respond with relief to a vote for Remain, while for most companies and their staff in Britain, business will continue as usual.

In Europe, Mr Cameron’s success will be welcomed. But relations with other EU countries will also change. His renegotiation deal will need to be enforced and passed by politicians in Brussels to deliver key reforms such as the “emergency brake” on paying benefits to EU migrants.

The European Parliament effectively has the power to veto Mr Cameron’s deal, and he will need to win their backing in the weeks after the referendum to deliver the reforms he has promised.