A new proposal for a new high-speed railway passing both Gatwick and Heathrow has been developed to enhance other major infrastructure projects for the south east.

Engineering consultancy Expedition today revealed its £10bn plan for HS4Air, which would connect the existing HS1 rail line with the planned HS2 along a route that passes via London's biggest airports.

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Alistair Lenczner, director at Expedition leading the development of the HS4Air proposal, said discussions are currently ongoing with a number of interested parties, spanning both national and regional bodies.

How would the M25 for high-speed trains work? 140km long between its connections with HS1 at Ashford and HS2 near Denham

A fifth of the railway will run in tunnels to avoid too big an environmental impact

Around 40 per cent of the route re-uses the existing Network Rail railway between Tonbridge and Ashford

He said: "HS4Air has been developed to allow rail and aviation infrastructure projects in south east England that are currently unconnected to become joined-up. This will offer greatly enhanced benefits for users and provide better value for the investments currently being made in the UK's strategic infrastructure."

Lenczner added:

In a way HS4Air can be regarded as a high-speed railway version of the M25 around London, except that it allows much faster journey times with no congestion and with far less impact on the environment.

The planned railway would provide fast direct access to both Gatwick and Heathrow airports from major UK cities to the north and west of London. Rail passengers would be able to travel to both on "fast regular services" from cities such as Birmingham, Manchester, Leeds and Cardiff without needing to switch trains.

Expedition says it will also slash journey times for passengers travelling between places south of London and towns in the Midlands. It wants a 15-minute surface transfer shuttle between Gatwick and Heathrow using high-speed shuttles between airport stations on the HS4Air railway.

Lenczner said the proposal was an example of integrated strategic planning that spans across multiple infrastructure sectors that "are too often planned within their separate 'silos'".

Expedition envisages the proposal alleviating pressure on the M25 and the number of domestic flights involving Heathrow and Gatwick.