Conservative plans to scrap a third runway at Heathrow in favour of a high speed rail line from London to Birmingham and Manchester have been condemned as “total nonsense” by the Government.

Transport Minister Lord Adonis insisted the West Coast Main Line could cope with passenger numbers until the mid 2020s. The Government was “keenly interested” in proposals for a new high-speed link, which would cut journey times between Birmingham and London by 35 minutes, he said.

He added: “This need to be a calm process of deliberation. It can’t be done without proper analysis.”

Tories unveiled plans for a new high speed line at their party conference in Birmingham last month. By linking Birmingham, Manchester and Leeds with St Pancras, the London station used for Eurostar channel tunnel services, it would provide a gateway to the Continent, they said. The line would provide an environmentally-friendly alternative to the proposed third runway at Heathrow Airport by providing travellers with high speed rail links instead.

But Lord Adonis insisted there was no need to rush to make a decision on a project that could cost more than £20 billion, especially as improvements to the existing West Coast Main Line, which also links Birmingham with London and Manchester, are due to be completed this year.

He said: “With the completion of the West Coast Main Line project, based on traffic growth figures, that will be able to meet capacity until the mid 2020s. And New Street station will provide capacity for two and a half times the current numbers once it has been refurbished.”

“We are keenly interested in looking at high speed rail lines.” He added: “It is total nonsense to link any decision about a high speed rail line with a third runway at Heathrow.”

Only a tiny proportion of Heathrow flights were between Manchester or Birmingham, and these were the ones that might be affected by the creation of an alternative rail line, he said.

During a visit to Birmingham, Lord Adonis met staff at the Highways Agency National Traffic Control Centre in Quinton, where he discussed the pilot scheme on the M42 which allows drivers to use the hard shoulder. He was also briefed on the New Street refurbishment scheme by Network Rail and the city council.