A final decision on the controversial HS2 high-speed rail line from London to Birmingham is being delayed until the New Year.
Plans to bore a £500 million tunnel to minimise impact on the Chiltern Hills landscape are being considered by Transport Secretary Justine Greening.
The 100-mile HS2 rail link, due to be built between 2016 and 2026, will cut the London-to-Birmingham journey time to 49 minutes.
However, the £32 billion project has provoked fury among many Tory MPs - including Welsh Secretary Cheryl Gillan, who has reportedly threatened to resign over the issue.
Ms Greening had been expected to make a final announcement before parliament rises for its Christmas break.
The decision has now been delayed until January as she has asked for a feasibility study to be carried out on a £1.5 million tunnel near Amersham, as well as an environmental probe.
The money would come from within the existing budget, as savings have been identified from other parts of the route.
"By accepting that the current proposals on the table are not right, the government has opened up a real opportunity to get this vital project right," said Shadow transport secretary Maria Eagle.
"It's a welcome start but ministers should now go further, stop being dogmatic and use this pause to finally take up our offer to work together on a long-term strategy for both high speed rail and aviation."