Conservative MP Louise Mensch is to quit the Commons and move to the United States with her family, she has announced.
The backbencher, who shot to prominence as a member of the Culture, Media and Sport Select Committee which investigated the phone-hacking scandal, said she was "devastated" but that it was a "necessary decision".
The move will give rise to a by-election for her seat in Corby, Northamptonshire, a tight marginal which will prove a major test for David Cameron's Tories.
Mrs Mensch, who was only elected in 2010, has carved out a high profile for herself and had been tipped for advancement at Westminster. But her husband, Peter Mensch, manager of rock band Metallica, lives in New York and she has been forced to divide her time between the UK and US.
In her resignation letter to the Prime Minister, mother-of-three Mrs Mensch said: "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. It is only through your personal intervention, delivered quietly and without fanfare, that I have been able to manage my duties for this long."
Mrs Mensch told Mr Cameron that letting her work from her constituency on Thursdays and Fridays had allowed her to spend more time with her children. She said: "Unfortunately, it has not proved to be enough. I am very sorry that, despite my best efforts, I have been unable to make the balancing act work for our family."
The Prime Minister replied that he was aware it had been "a particularly difficult period for you and your family".
Mr Cameron said Mrs Mensch had been an "inspiring" MP and hinted that she had been in line for a role in the Government: "It goes without saying that I had wished to see you serve for longer and at a more senior level. But you have made a tremendous contribution in the short period you have served in the House of Commons, and have much to be very proud of."
Labour's prospective parliamentary candidate for Corby and East Northamptonshire, Andy Sawford, said: "Louise Mensch was obviously struggling to balance being MP for Corby and East Northamptonshire with her family and business commitments. I respect her for the honest way she talked about this and her decision to step down.
"Labour is now looking forward to the campaign ahead, fighting for action on jobs and the economy, supporting our local services, which are facing massive cuts, and working with local community organisations."