The British PM's worst nightmare has just happened...
The UK has voted to leave the European Union, he has lost the referendum campaign and his authority as Prime Minister is unlikely to ever recover.
The Prime Minister's future is still uncertain but one thing is for sure: he would never have called the vote in the first place if he thought he would be on the losing side.
There will be pressure on Mr Cameron to resign immediately and some Conservative backbenchers have already said privately that they think he should go.
Reports in the British media suggest that Home Secretary, Theresa May will take over as a caretaker leader to 'steady the ship.'
Unlike some of her cabinet colleagues, she has managed to avoid the majority of the mudslinging in the pre-referendum debate.
If Mr Cameron does decide to resign a leadership contest will follow and the old wounds between Remainers and Brexiteers are likely to reopen.
There is an argument, however, that what the UK needs more than ever right now is stability.
Some MPs - including Boris Johnson - signed a letter before the outcome of the vote saying whatever the result of the referendum Mr Cameron should stay on as Prime Minister. Now that the UK voted to leave the EU, that support will be put to the test.
It is almost inevitable that a reshuffle will follow as the Prime Minister desperately tries to reunite a party bitterly split after a bruising referendum campaign.
His choices - whether driven by revenge or reconciliation - will prove crucial. To put it bluntly, if he gets it wrong Mr Cameron’s authority may be too weak for him to hang on to the job much longer.
Whatever the current inhabitant of No 10 decides to do in the short-term, in the long-term his Downing Street days are numbered.
The next couple of years are likely to be dominated by complex renegotiations that are part of the uncoupling of the UK and the EU.
When the country finally emerges from those crucial negotiations, it's highly unlikely that the same British Prime Minister will be in power.