Sources close to the Prime Minister, David Cameron, are saying that he will "sit tight" in Number 10 after the General Election even if he lacks a majority in parliament.

Mr Cameron says that, if he leads the largest party after this week's poll, any attempt to outvote him by Labour and the SNP would be "illegitimate". Boris Johnson has warned of "Ajockalypse Now" if Labour try to lead a government with the support of SNP MPs.

This rhetoric is irresponsible and will come back to haunt the PM. There is simply no way, short of coup, that Mr Cameron could remain in office if the Tory legislative programme in the Queens Speech is rejected by a majority of MPs in the House of Commons.

As the former Cabinet Secretary, Lord O'Donnell, has made clear along with other constitutional authorities including Professor Vernon Bogdanor, the largest party does automatically become the Government in our system. It is the leader who commands the largest number of seats in the House of Commons who gets the keys to Number 10.

However, Mr Cameron is right to this extent: the sitting Prime Minister does have the first attempt to form a new government in a hung parliament.

He will try to revive his coalition with the Liberal Democrats, though Mr Cameron's insistence on a referendum on British membership of the EU may be a problem.

But even if he succeeds, arithmetic could be against him. If the opinion polls are accurate, it is likely that the combined forces of Labour and the SNP, along with smaller parties such as Plaid Cymru, the Greens and the SDLP, will prevent Mr Cameron's Queen's Speech securing a majority. That would effectively kill his administration.

Mr Cameron may then claim that Ed Miliband has broken his pledge not to do any "deal" with the SNP. The Labour leader will insist that this is not the case and that he has not offered Nicola Sturgeon anything in return for her support.

Of course, by then the LibDems might well have decided to join a minority coalition with Labour, realising that the Conservatives simply lack the numbers to form a stable government.

If voters have difficulty following all of this, it is not surprising. Our advice is this: ignore 90 per cent of what is being said about the post-election period in coming days.

Only after the country has made its choices clear on the morning of May 8 will the parties begin seriously to seek a governing coalition that best reflects the result.

Part of the problem is our outdated first-past-the-post electoral system. It assumes elections are a choice between only two parties. We now have a multi-party democracy and we need an electoral system that reflects this.

The SNP could be in line to win nearly all of Scotland's seats on less than half of the popular votes. That would be impossible for Holyrood, which has a proportional system that ensures the number of seats a party wins in parliament reflects its share of the vote.

But that is for the future. At present, voters should simply set aside the red lines and ultimatums and vote for whoever they think is the best candidate to be their MP. Keep calm and carry on voting.