Andy McSmith: Why Mensch's move has gifted Labour a by-election win

Corby was a Labour seat from 1997 to 2010, when Mensch snatched it off them with a majority of just 1,951

Louise Mensch claimed yesterday to be "overwhelmed" by messages of support – which suggests she is not well attuned to the private thoughts of David Cameron or the party activists left fighting to retain her seat.

It is rare for an MP to resign in mid-parliament because they've opted for a life outside politics. Many other MPs have made that life choice, but tend to leave it until the next general election to step down, out of consideration for their electors and party colleagues.

Mensch has almost certainly gifted the Labour Party a high-profile by-election win. Labour's prospective candidate, Andy Sawford, was on holiday when she dropped her bombshell, but he ought to have an easy passage into the Commons once the by-election campaign starts .

Corby was a Labour seat from 1997 to 2010, when Louise Bagshawe – as she was then known – snatched it off them with a majority of just 1,951. Then, the advantages were all with the Conservatives – not just nationally, where David Cameron was pitted against an exhausted Gordon Brown, but locally too. Corby's sitting Labour MP, Phil Hope, had repaid more than £41,000 to limit the damage caused by the expenses scandal, while Louise Bagshawe appeared to represent a clean break from everything people disliked about politics and the old "nasty" Conservative Party.

But she's now shown her life outside politics is more important to her than the town she claimed to be so proud to represent. It is many years since a by-election's been held in these circumstances, but precedents from the 1980s suggest voters will use the occasion to punish the party. No surprise Boris Johnson has moved with the speed of Usain Bolt to kill any suggestion he'll agree to be the Tory fall guy in Corby.