Bold plans for a new high-speed rail service which could take passengers from Ashford to Gatwick airport in just 25 minutes have been rejected.

The new £10 billion railway service - dubbed the 'M25 for high-speed trains' - would have connected the existing high-speed rail line in Ashford to the planned HS2 project along a route that passed via both Gatwick and Heathrow airports.

But the Department for Transport (DfT) has turned down the proposal, which was submitted by engineering consultancy Expedition earlier this year after the DfT called for ideas for a new southern rail link to Heathrow.

The scheme - called HS4Air - would have linked Kent to major cities including Birmingham, Manchester and Cardiff – without taking passengers through the capital.

Bosses were planning to upgrade the existing train line between Ashford International and Tonbridge to cater for the high-speed service.

Part of the 140km-long line would have run in tunnels to avoid adversely impacting on environmentally sensitive areas such as the Surrey Hills.

Expedition director Alistair Lenczner says the company is “disappointed and perplexed” the plans were rejected.

He said: “We got a response from the DfT saying it didn’t want to engage any further without allowing us to demonstrate a business case.

“It said it was concerned about the affordability and that it would likely face issues because it is in the greenbelt, which is true of any major infrastructure.

“Most of it was going to be in tunnels and the impact to open green areas would have been less than the Lower Thames Crossing.

“We’re trying to encourage people to get out of cars and use more sustainable modes of transport and the HS4Air would have contributed to that.

“We have had lots of messages of support who are also utterly gobsmacked that it has been rejected at this stage.

“We are perplexed and disappointed but we don’t intend to back down."

Mr Lenczner says the engineering company plans to challenge the decision further with the DfT.

He added: “For us it is a premature rejection of proposals without allowing us to show evidence.

“We will go back to them and ask why they have teared it down without any evidence and would be interested to find out something specific.”

When the plan was first revealed, Mr Lenczner said the proposal would alleviate pressure on the M25 and the number of domestic flights involving Heathrow and Gatwick.

The plans - which would have also connected the railway line to the Great Western Main Line - included a shuttle service on the line between both airports, allowing passengers to transfer in just 15 minutes. It was not known who would fund the scheme.

A DfT spokesman said: “The call for ideas sought market led proposals to enhance our railways which were financially credible without government support.

"The HS4Air proposal did not meet the requirements as set out in the call for ideas.”