A final decision on whether to go ahead with the controversial HS2 high-speed rail line between London and Birmingham is being delayed until the New Year.
Transport Secretary Justine Greening is understood to be considering whether a £500 million tunnel should be bored to minimise impact on the Chiltern Hills landscape.
MPs along the route - the majority Conservative - have been some of the fiercest critics of the scheme while residents on the proposed route through the Home Counties and through the Midlands have mobilised vocal opposition groups.
Welsh Secretary Cheryl Gillan has reportedly threatened to resign over the issue.
The £32 billion, 100-mile rail link, due to be built between 2016 and 2026, aims to cut the London-to-Birmingham journey time to 49 minutes.
Ms Greening had been expected to make a final announcement before parliament rises for its Christmas break.
But 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.
Shadow Transport Secretary Maria Eagle said: “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.
“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.”