The government's latest business case for the HS2 high-speed rail link has lowered the amount of benefit it predicts relative to the cost.
The expected benefit-cost ratio (BRC) has fallen from £2.50 to £2.30 in benefits for every pound spent.
That fall is mainly due to a £10bn rise in the scheme's projected £42.6bn cost, which was added earlier this year.
The report is the latest update on questions such as who benefits and by how much. It also has revised an earlier assumption that business people do not get much work done on trains, a view that was widely criticised at the time.
The new study has cut by one-third the value put on saving an hour's worth of time getting between meetings or workplaces on a quicker train, to reflect that productive work is also done while travelling.
One part of the report argued that the alternative to HS2 would mean 14 years of route closures and longer journeys. A study, prepared by Network Rail and the management consultancy Atkins for the government, said that without the project, there would have to be 2,770 weekend closures on the East Coast, West Coast and Midland main lines for the same intended capacity of HS2.
Transport Secretary Patrick McLoughlin said without the new line, the West Coast, East Coast and Midland Main Lines were likely to be overwhelmed.
He said it would also bring benefits for regional and commuter services, as well as increasing the amount of freight that could be carried by rail.
But he warned it needed broad political consensus or it would end in nothing: "You can't play politics with our prosperity. The new North-South line is a multi-billion, multi-year investment in the future of Britain."
Secretary of State for Transport Patrick McLoughlin said: "We need a radical solution and HS2 is it. A patch and mend job will not do – the only option is a new north south railway.
"HS2 brings massive benefits to the north, is great for commuters and the alternatives just don’t stack up.
"Now is the time to be bold and deliver a world class railway which Britain deserves and can truly be proud of. Future generations will not forgive us if we fail to take this opportunity."
