Boris Johnson told to announce his intentions for next election

Friends of Boris Johnson say he thinks it is possible to combine the job of mayor and MP. Photograph: Richard Kaminski/REX

Conservative party headquarters is putting pressure on the Londonmayor, Boris Johnson, to announce shortly that he will stand as a Conservative candidate at the next election, hoping to prevent another bout of speculation about his intentions overshadowing the party conference this autumn.

Friends of Johnson responded to the latest speculation by saying he had still not made up his mind, but that he thinks it is possible for him to combine the job of mayor and MP.

But his allies are clear there is a limited window of opportunity for him and that it would be regarded as "poor form" for him to hold back until the autumn, when David Cameron and George Osborne will be wanting the focus to be on the Tory election policies rather than speculation about future party leaders.

Tory constituencies in safe seats have also told Central Office they would like an indication from the mayor on whether he is willing to put forward his name.

With the London local elections, as well as the European elections dominating the political landscape until early June, and August in effect dead to politics, Johnson will feel obliged to make a decision in June or July. September remains an outside possibility.

A decision to stand would be genuinely welcomed by Downing Street, but only so long as the terms and timing of the announcement are agreed with Cameron. The two seats with which Johnson has been most frequently linked are North West Hampshire, or Louth and Horncastle.

Labour is preparing to mount a scathing attack on Johnson as a part-time mayor and part-time MP if he decides to become an MP before his tenure as London mayor ends in 2016. It is likely to damage his brand if he is seen to be scrambling into parliament in time to be present for any leadership succession battle in the wake of a general election defeat.

Sadiq Khan, the shadow justice secretary and potential Labour candidate for mayor in 2016, claimed: "Johnson has stopped hiding the fact that he is a part-time mayor. The greatest city in the world needs a full-time mayor committed to London. Johnson has simply lost interest.

"He has no vision for the future of London but only cares about his own future as leader of the Conservative party.

"He will leave no legacy whatsoever to young Londoners or his successor as mayor. If the Tory party really are considering him as their next leader, I wish them well."