Transport secretary Justine Greening is considering a string of concessions to appease protestors and Tory MPs, including a £500m tunnel to reduce the proposed network’s environmental impact.

A decision had been expected on 20 December, but the transport secretary will this week announce a delay to consider changes to designs, the Sunday Times reported.

The 1.5 mile tunnel near Amersham in Buckinghamshire would reduce the environmental impact in the constituency of Welsh Secretary Cheryl Gillan, who has previously hinted she was prepared to quit the cabinet over the railway.

Savings from redesigning some of the deep cuttings along the route will help to pay for the extra tunnelling, although it is understood there are no changes to the planned route.

The HS2 line is timetabled to be built between 2016 and 2026 and will be the flagship of the Government’s heralded transport infrastructure investment plans.

The government’s proposal is for a new Y-shaped high-speed rail network. The initial phase would connect London and Birmingham, reducing journey times to 49 minutes from 84. Lines to Manchester and Leeds would open by 2033.

Behind the scenes the complex procurement process is advancing. Turner & Townsend, Mace/Atkins, CH2M Hill and Parsons Brinckerhoff are understood to have been shortlisted for the high profile role of development partner, worth up to £70m.

The winning bidder wil help to develop the Hybrid Bill, which is due to go before parliament in Autumn 2012.

A host of professional firms have also submitted pre-qualification questionnaires for the even larger professional services framework

The civil engineering and structural design packages alone are worth up to £200m, with separate railway and environmental consultancy work worth around £50m each.