Chancellor George Osborne and his officials held a string of private meetings with the backers of plans for a new airport in Kent, including the London Mayor Boris Johnson, it has emerged.

The previously undisclosed contacts stretched over a four-month period between May and August last year, according to details obtained by the KM Group under the Freedom of Information Act.

The government has refused to say why the meetings were held and who asked for them, saying that it is not in the public interest to do so.

It did reveal that Mr Osborne held a face-to-face meeting with Boris Johnson in July last year to discuss his plans for an island airport - dubbed Boris Island.

But the Treasury has refused to say exactly what was discussed at the meeting because it involved the formulation of government policy.

The meeting will reinforce speculation that the Treasury and the Chancellor have been instrumental in pushing the government towards backing the highly contentious idea of a new hub airport after David Cameron had, a year earlier, publicly vetoed the prospect.

Those suspicions are underlined by the disclosure that Treasury officials also had regular contacts with Foster and Partners and Halcrow, the backers of an alternative Thames Estuary airport scheme.

Four meetings took place between May and August last year, with Treasury officials being hosted for one of those meetings at the London headquarters of Foster and Partners.

In November, Lord Foster unveiled his company’s vision for a new £50bn international hub airport in Kent, and last month the government announced it was consulting on the idea as part of a review of aviation capacity.

But it seems the Treasury was actively involved in listening to the case for an airport several months before. The first meeting officials had with representatives of Fosters and Partners was held in May, followed by a second just a month later.

After the first meeting, Foster and Partners sent an email which said: "We found the conversation we had both stimulating and highly reassuring as you both made points we passionately believe in."

This meeting was followed by two more in July and August, this time involving both Foster and Partners and Halcrow, infrastructure consultants working on the scheme.

The government has refused to release any information about what was discussed at any of these meetings.

In its response to our FOI request, the Treasury confirmed "the issue of an Estuary airport was covered to an extent" but said there were "weighty arguments" for withholding the information.

The response said: "There is a strong public interest in effective policy development.

"If ministers and officials were not able to conduct a free and frank discussion it would result in people providing less candid opinions and therefore impact on the quality of policy development.

"We do not think this would be in the public interest, particularly as this is currently a very live issue."