Under the Act, a Dissolution can occur only under one of the two scenarios spelled out in section 2. Events which would normally have been matters of confidence, such as losing Second Reading of a Finance Bill, the Estimates, or even the vote on the Queen’s Speech, are not covered. It may be said, of course, that a government in that position would need to seek the endorsement of the Commons in a vote of confidence. But there is no requirement to do so and, if the Opposition of the day did not want to deliver the coup de grâce (or could not do so) then a government could limp on – in theory, for the remainder of its term.
So what might be the effect of the Fixed-term Parliaments Act were there to be a vote on 23 June to leave the EU? Here the crystal ball clouds over very rapidly.
A narrow result (say 51-49) on a low turnout (say less than 50%) might fuel demands for a second referendum. This would not be a popular course, to say the least; but a complicating factor is the substantial Remain majority in the present House of Commons. There might be no shortage of Remainers who might want to throw the “sovereignty of Parliament” argument back at the Leavers.
But it is at least possible that a case could be made for a second referendum: not a replay to get a different result – although such things are not unknown in EU affairs – but to approve an exit package. Both the holding of a referendum, and the nature of an exit package, might be seen as matters of confidence – but of course would not be covered by the Act.
Another factor might be a differential result in two or more parts of the United Kingdom – for example, a vote to leave in England, but to remain in Scotland, which would complicate the Parliamentary arithmetic. And if a substantial victory for Brexit resulted in the formation of a new administration, and Parliamentary approval were needed at various stages of withdrawal, the denial of that approval would be seen by many as a matter of confidence, but again would not be covered by the Act.
Perversely, though, the operation (or non-operation) of the Act might reflect the will of the House of Commons, because of the wholly unpredictable effect that a Brexit General Election might have on the political map of the United Kingdom. As I say, the crystal ball clouds over.