In defiance of Michael Heseltine's recent advice  to him not to stand for parliament until he has completed his second term as Mayor of London in 2016, Boris Johnson is set to announce that he will seek to return as an MP at next year's election. The Daily Mail  and the Sun  report that he will confirm his intentions by the summer in order to avoid acting as a distraction at the Conservative conference in October (although a distraction he will doubtless remain).
There is no constitutional bar to Boris becoming an MP and continuing to serve as Mayor. Indeed, there is a precedent. After the first contest in 2000, Ken Livingstone remained the MP for Brent East until 2001. But if, as he has long hinted, Boris intends to run for the Conservative leadership in the event of a Tory defeat in 2015, things become more complicated.
It is inconceivable that he could serve as both Conservative leader and Mayor of London for any significant period of time, leaving him with three options: to avoid standing in 2015, to persuade the party to delay any contest (if Boris stands down a minimum of six months before the end of his term, his deputy takes charge), or to trigger a mayoral by-election. Of these three, despite his pledge to serve a full second term in City Hall, it is the latter that is most likely. But the move would likely do significant damage to his party, which would be accused of disrespecting the mayoralty and would struggle to avoid defeat in the capital. If the polls in 2015 continue to suggest that Boris would outperform any alternative leader, this might be regarded as a price worth paying, but it is one the Tories should start to weigh up now.