Mensch resigns: Cameron faces dangerous by-election

The prominent Tory MP Louise Mensch resigned this morning, in a move which shocked Westminster and could damage David Cameron.

The Corby MP said she was stepping down to spend more time with her family.

"As you know, I have been struggling for some time to find the best outcome for my family life, and have decided, in order to keep us together, to move to New York. With the greatest regret, I am thus resigning as a member of parliament," she wrote to the prime minister.

"It is only through your personal intervention, delivered quietly and without fanfare, that I have been able to manage my duties for this long."

Mensch's decision to stand down robs Cameron of another of his A-list candidates and deprives the Conservative party of a prominent spokesperson who had some recognition on the public stage.

In the short term, it also triggers a November 16th by-election in a seat which could easily swing to Labour. Corby has been held by Labour since 1997 before Mensch squeezed in with a majority of 3.6% – some 1,951 votes.

Ed Miliband will be determined to win back the seat to show he can advance Labour's electoral prospects during a difficult period for the Tory party.

"I know that this has been a particularly difficult period for you and your family and that this will not have been an easy decision for you to make," Cameron wrote to the backbencher.

"With that in mind, it is with enormous regret that I accept your resignation. I do so only because I wish to support you in acting in the best interests of your family, which must come first."

The resignation triggered frenzied speculation among Tory supporters that Boris Johnson could go for the seat, with an eye to a future leadership bid.

The London mayor is unlikely to take that option, however. Although he has instructed staff to prepare the ground for a future leadership attempt, the moment is too early into his second term at City Hall

Mensch has been a colourful and influential figure on the Tory benches in her short period as an MP. The former chick-lit author, who is married to Metallica's manager, admitted taking class-A drugs, focused internet attention on the treatment of prominent women in public life and started up her own social media site.

Her most significant moment came during the media committee's inquiry into the phone-hacking scandal, which boosted the reputation of select committee and saw News International employees accused to lying to parliament.

There, she became close allies with Labour MP Tom Watson, whose reputation was also boosted by the committee's work.

"You're a remarkable character Louise," he tweeted today. "Be all you can be."

The exchange with the prime minister suggests Mensch has been struggling to keep her various interests going for some time. The fact she was given more time away from parliament than whips afford most MPs helps explain her absence during recent key votes.

Speaking on Question Time recently, the MP admitted she suffers anxiety attacks because of drug taking in her earlier life. "It is something that I regret incredibly, that in my youth that I messed with my brain," she said.

"I said we all do stupid things when we are young. It’s had long-term mental health effects on me. It’s caused me to be more anxious than I need to be."

The combination of her background, political aspirations, business interests and a tran-Atlantic relationship appear to have proved too much for Mensch, who said she would spend the week with her family.