Via the explanatory notes to the Fixed-term Parliaments Bill, this took some wrestling with:
Clause 2: Early parliamentary general elections
15. Clause 2 provides for the circumstances in which an early parliamentary general election can be held, that is, an election with a date of poll which must be earlier than the date of poll for an election under clause 1. Subsection (1) provides for an early election if the House of Commons passes a motion that there should be such an election. Where such a motion has been passed on a division, the number of members who voted in favour of the motion must be a number equal to or greater than two-thirds of the number of seats in the House, including any vacant seats. It is for the Speaker to certify that such a motion has been carried and whether or not it has been passed on a division of the House, and if so by the required majority.
16. Clause 2 also provides that, if the House of Commons passes a motion of no confidence in the Government, an election must be held unless within the period of 14 days the House passes a motion expressing confidence in a Government. The intention is to provide an opportunity for an alternative Government to be formed without an election. Again, it is the Speaker who will certify whether the motions in question have or have not been passed.
I am persuaded that, in combination, these measures provide an appropriate balance between political stability and the facility to dismiss an incompetent government. A simple majority for an early election was attractive, but of course the power of the Whips to achieve a majority is such that it would render meaningless the apparent transition from the PM deciding to Parliament deciding. A clear 2/3 seems to me justifiable.
We will see.