Justine Greening, the transport secretary, is expected to announce this week that she will be delaying any final decision on the controversial HS2 project until the new year. The proposed high speed train link between London and Birmingham could now feature a tunnel, estimated to cost around £500 million. The structure would be built in the Chilterns, where there is opposition to the scheme.

The tunnel would be 1.5 miles in length, and would pass through the constituency of Cheryl Gillan, the Welsh Secretary. She said if HS2 was to cut through Chesham and Amersham, she would consider stepping down from the cabinet.

Other Tory MPs opposed to the scheme include Government whip Jeremy Wright, Europe minister David Lidington and Speaker John Bercow. The £32 billion rail project is designed to cut the journey time to Birmingham from London down to less than 50 minutes. Supporters of the scheme say it will result in economic benefits for the Midlands.

Detractors claim that there is no proof that the project would return anywhere near the amount it would cost. Sources claim that the money needed to build the tunnel would come from savings made on other parts of the route. According to the Campaign to Protect Rural England, the project would need to consist of additional tunnelling if the Chilterns were not to be adversely damaged.

Plans have also been introduced by Labour which would involve the HS2 route following existing transport corridors. Maria Eagle, transport spokesman, said that not considering the Labour plans could now cost the tax payer an extra £500 million.