Confusion about Boris Johnson's future in Oxfordshire was growing last night as local Tories appeared to cast doubt on his pledge to step down as Henley MP "very soon" if elected London mayor.

The chairman of Henley Conservative Association, John Walsh, told the Oxford Mail he hoped Mr Johnson would "keep things ticking over for a period of time" if elected mayor later today, before adding: "We haven't decided how long that period will be."

Mr Walsh went on to say that in the event of a Johnson victory in the capital, the local party would hope to select a new candidate for Henley "before the summer".

It followed separate comments in a national newspaper, in which Mr Walsh raised the possibility of "an autumn or spring by-election".

Mr Johnson's spokesman failed to return calls to the Oxford Mail yesterday, but if Mr Walsh's timetable is correct, Henley residents face the prospect of their MP juggling two jobs for up to a year.

In March, Mr Johnson said he would have to resign from Parliament shortly after beating Ken Livingstone, triggering a by-election.

He told BBC Radio: "Obviously I have said throughout this I can't conceivably do both things. There would of course be a by-election very soon afterwards."

The MP has previously maintained the timing of any resignation is "in the hands of the (Henley Conservative) Association and the party".

Yesterday, Wantage MP Ed Vaizey predicted Mr Johnson would win because of a growing feeling that Londoners wanted "a change".

He said: "Boris will be different. He genuinely wants to make a difference to the three things that matter most to Londoners - crime, transport and their environment.

"And he'll do it in his own inimitable style."