Boris Johnson came under heavy fire at a heated pre-election 'debate' for hoping to juggle his role as London mayor with being an MP for Uxbridge and South Ruislip.

Rival parliamentary constituency candidates led the attack, saying he would not be able to adequately perform both functions at once.

But the Conservative candidate blasted back: “I do have a record as a previous constituency MP and I worked flat out for those people.”

Addressing a packed audience, he added: “I want to try to allay any anxieties that any of you might have about how accessible I would be, or how present I would be.

“I want you to know that I'm required by my party to be as useful as I possibly can to the whole campaign, and I hope that you will forgive me for that, but unless I specifically tell you otherwise, I am campaigning here in Uxbridge and South Ruislip, to improve the lives of everybody in this constituency.”

The remarks were made at the constituency's second hustings, held at St Margaret's Church, in Windsor Street, Uxbridge, on Friday (April 10).

For two hours, candidates from the five main political parties answered questions from the audience, on everything from Heathrow Airport's proposed expansion and HS2 to religion's role in politics and the Israel-Palestine conflict.

On most issues the candidates repeated their parties' official positions – a notable exception being when Labour's Chris Summers vowed once again to vote against HS2, rather than toe his party's line.

Another highlight came when Mr Johnson admitted he still held hopes for resurrecting his plans for a 'Boris Island' airport hub on the Thames Estuary, which Mr Summers claimed would inevitably mean closing Heathrow Airport, costing countless jobs – a charge Mr Johnson dismissed.

Mr Johnson said he was against a third runway as it would be “not deliverable”, adding: “That's why if we want to compete, if we want to communicate with the rest of the world, to get this country forward, then we do need to start thinking about serious and practical alternatives, and that's why I'm sticking to my guns.”

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The hustings' integrity was thrown into question at the start of the meeting, when independent candidate Michael Doherty, who was not allowed on the panel, left to the back of the room, saying: “That's democracy at work here in Great Britain.”

A Brunel University student from the audience continued this theme later in the evening, blasting Hillingdon Council for not housing a polling station on campus.

Outside, meanwhile, taxi drivers protested with a banner reading “Totally failing London”, calling for Mr Johnson to be sacked.

It was during the candidates' closing remarks, however, that Mr Summers, Liberal Democrat Mike Cox, UKIP's Jack Duffin and the Green Party's Graham Lee gibed at Mr Johnson for his ambition to manage two political roles simultaneously.

Mr Summers said: “You don't want a candidate who's already got mayor of London, which is one of the biggest jobs out there, who's also an author, a newspaper columnist, numerous other sidelines… You're looking for someone to represent Uxbridge and South Ruislip and stand up for you on all the issues that matter. You're not looking for a dinner party guest.”

Mr Cox added: “We need somebody who is going to work hard for you irrespective of fear or favour, party or personal interest and, yes, God forbid, a burning ambition to be prime minister.

“If you want an MP to be a national celebrity, with no moral compass, who parachutes himself in from time-to-time, then vote for that man over there.”

The hustings was one of three being held for the constituency ahead of the General Election on May 7 – the first having already taken place at Brunel University.

Friday's was the only such meeting that Mr Johnson will attend. It differed from the others in that the format did not allow for any back-and-forth or debate between candidates and the questions were all pre-submitted.

The third and final hustings for the constituency will be held at Yiewsley Baptist Church, in Colham Avenue, on April 26.