Boris Johnson claimed his decision to run for London Mayor could lead to him spending more time in Oxfordshire.

The Henley MP, confirming he would seek the Conservative nomination for next year's mayoral race, insisted he could serve his constituents at the same time as battling it out with Labour's Ken Livingstone in the capital.

In an attempt to placate local supporters, who had urged him not to turn his back on them, Mr Johnson said he had "freed up" time to dedicate to his constituency.

But with signs of a backlash emerging, with one Tory accusing the MP of putting his career ahead of his obligations to his constituents.

Mr Johnson, in an interview with the Oxford Mail, said he was stepping down from the Tory front bench and would be scaling back his lucrative after-dinner speaking engagements.

He said: "I have given up my position as shadow higher education spokesman and I will not be doing any more speeches in Little Wittering.

"I have freed up some time and I think I'll be in Henley just as much as before, if not more. I have knocked on the head the other things that were getting into my life."

Asked whether he was concerned that people who voted for him at the General Election two years ago might feel let down, Mr Johnson said: "I hope not. I hope they understand they will get as much value out of me, if not more, as before."

It is understood Mr Johnson would give up his Commons seat only if he was elected mayor. If he was unsuccessful, he would seek to carry on as MP for Henley. The MP said his decision had been a "huge wrench" but said he had received a "massive amount of support in Oxfordshire, from amazing numbers of people".

John Walsh, chairman of the Henley constituency Conservative Association, gave Mr Johnson "full backing".

But Pearl Slatter, a South Oxfordshire district councillor, said: "I'm terribly disappointed. He's let us down. We worked very hard to get him elected in a very safe seat. I thought he was different to all the rest, but it seems he's not, and he's serving his own career."

Tony Crabbe, Tory county councillor for Benson, said: "We would be sorry to lose Boris but it's entirely his decision. He has done a good job as MP.

"I think he would have great difficulty in doing both jobs properly.

"I'd be sorry to see him go."

Mr Johnson will now have to win over London voters to become one of five shortlisted Tory candidates who will then do battle at a series of hustings before a final candidate is announced in September.

The mayoral election takes place next May.