Secret plans could see a four-runway airport built near Oxford instead of in the Thames Estuary, it is claimed.

The Independent on Sunday claims to have uncovered confidential documents by a business consortium preparing to declare plans for a new airport west of London.

The documents reportedly say the airport would have to be within half an hour's reach of London, either on the Great Western Main Line to Bristol or the High Speed 2 link to Birmingham.

The newspaper claims a "world-leading infrastructure firm" is examining flat tracts of land such as near Banbury, Oxfordshire, or Twyford, Berkshire.

If true, the plan would be in direct competition to two of the leading ideas for a new airport in the South East - both of which are in the Thames Estuary, angering political leaders in north Kent.

One is Lord Foster's plan for a hub airport on the Isle of Grain, pictured above, while the other is Boris Island, a floating airport in the estuary itself.

An unnamed source told the IoS: "What this idea does is put people's pipedreams, like Boris Island, to one side and shifts the political debate away from Heathrow."

The claims reflect a ramping-up of political pressure in Westminster ahead of the government's formal consultation on air strategy.

Conservative back-bencher Tim Yeo made a dramatic U-turn this week to support a third runway at Heathrow, challenging David Cameron to be a "man or mouse" over making a decision.

Medway Council and Kent County Council are both against any plans for an airport in the Thames estuary.

Backed by environmentalists but opposed by the business group Demand Regeneration in North Kent, they are urging Whitehall to fill existing capacity at smaller airports instead.

A cross-party group of Medway councillors travelled to Birmingham Airport on Friday, accompanied by Rochester and Strood MP Mark Reckless (pictured right), Kent County Council officials and Medway Council's regeneration director, legal officer and media chief.

They met with senior managers at the airport, which wants to expand and be named a national facility on a par with Heathrow or Gatwick.