London Mayor Boris Johnson's plans for an airport off the north Kent coast remain on the agenda, according to his deputy.

Shadow transport secretary Theresa Villiers said the Conservatives were not actively pursuing the idea for the airport after announcing that a future Conservative government would scrap a third runway for Heathrow and build high-speed rail links to Birmingham, Leeds and Manchester instead.

But Sir Simon Milton, the Conservative deputy mayor, has insisted the scheme for an airport in the Thames Estuary remained on the agenda.

In an interview with the Kent Messenger Group, he said: "Regardless of the decision for high speed rail links, which is an excellent idea, we do still need to look at the airport capacity for London and the south east. That will not go away."

He said he understood why people in Kent might be alarmed but suggested those who opposed the idea were overlooking the jobs boost an airport would bring.

He said: "I can understand that people in Kent might be concerned to know what might happen.

"If an airport was built, there would be a massive boost to the economic regeneration and to jobs and some of the people who are most concerned would benefit.

"This is an issue for the long term and what we want to do is start a debate, given that we do not believe that Heathrow can expand beyond its existing capacity."

There has been widespread opposition to the idea in Kent across the political spectrum, with Kent County Council arguing it would be better to focus on developing Kent International Airport in Manston, Thanet.

Sir Simon said he would be happy for Manston to be considered as an alternative option.

He said: "We need to look at future capacity needs and where it would be sensible to expand.

"Manston should be considered as a possible option."

Labour MP for Sheppey Derek Wyatt welcomed what he said was a Conservative u-turn and the pledge to build high-speed links.