The proposal is contained in a final report to be submitted to the Department of Transport next week by Union Railways, the BR subsidiary responsible for the 108km (67- mile) link from the Kent coast.
Union Railways had originally wanted the final stretch of the pounds 2.5bn- pounds 3bn link to run overland along the existing North London Line from Dalston with a spur into St Pancras on the grounds that a tunnel would be too expensive.
But according to its final report the increased cost involved in tunnelling the final 3.25km (2.17 miles) through north London had come down dramatically from the initial estimate of pounds 20m- pounds 50m.
The report also suggests digging the tunnel 30-35m (33-38yds) below street level to allay fears about the effects on property.
Local residents have fought a vocal campaign to get the line built in a tunnel. But Union Railways engineers have also discovered that the costs of relaying the track bed on the North London Line and rebuilding local stations, together with disruption to existing commuter services, would be much higher than thought previously.
The Union Railways report says that building the terminus above ground at St Pancras would be pounds 350m- pounds 400m cheaper than the alternative of a new pounds 1.4bn subsurface station at King's Cross.
The St Pancras development would involve extending the station northwards with six platforms for international trains and four for Midland Mainline Services. The report also favours building a below- ground station for Thameslink services west of St Pancras as an integral part of the redevelopment.
Union Railways is against an intermediate international station at Stratford, east London. It favours a station at Rainham, Essex, or Ebbsfleet, Kent, as they are cheaper options, closer to the M25 and offer more development potential.
If ministers accept the proposals, the last 16.5km (10.2 miles) of the link would run in a tunnel from Rippleside, just east of Barking in east London, to St Pancras.
The prospects for Crossrail, the east-west cross-London rail link, receded when the Government persuaded the parliamentary committee considering the Bill to delay further work until the New Year.