Officials in the Department for Transport have now apparently found an extra ￡500m to construct a 1 ½ -mile tunnel under the Chilterns Hills, west of Amersham, to stop the line becoming a blot on the landscape. The money would come from within the existing budget, as savings have been identified on other parts of the route.

Opponents, who say the line would damage an area of outstanding natural beauty, have welcomed the news.

The new rail route would see trains up to 400 metres long leaving Euston, going straight into a tunnel to surface at an interchange six miles away at Old Oak Common Lane in North-West London.

At speeds up to 225mph, the new trains would travel to West Ruislip, across the Colne Valley to Amersham and then Aylesbury, to arrive at a new interchange station near the National Exhibition Centre in Birmingham.

The transport secretary, Justine Greening, is expected to announce a delay to the decision on the £32bn HS2 project. She had been expected to decide whether to approve the scheme before 20 December 2011 but the decision has now been delayed until mid-January to allow for a feasibility study in addition to a study of the environmental impact of the proposed route.

A Department for Transport spokeswoman said the secretary of state was expected to provide an update on the timings of the HS2 project shortly.

The decision to look again at the route follows a report by the Commons transport committee last month, which suggested another look at the impact and benefits of HS2.

The proposals have proved controversial among MPs whose constituencies would be most affected by the planned route.

The 100-mile rail link, which would be built between 2016 and 2026, aims to cut the London-to-Birmingham journey time from 1 hour 22 minutes to just 49 minutes.