Government and unions are still pushing hard for the HS2 north-south rail link after a report showed its economic benefit was not as high as previously thought.

The benefit-cost ratio (BCR) for the full two-phase project is now estimated at 2.3, down from a figure of 2.5 given in August 2012.

This means that for every £1 spent, the economic benefit of the entire scheme will produce a benefit of £2.30 compared with £2.50 estimated last year.

The government said the revised BCR was “similar to Crossrail and higher than the BCR ratio for some other major projects when approved, such as Thameslink and the Jubilee Line extension.”

It added that the ratio would increase to 4.5 if rail demand continued to rise until 2049.

The new figure appeared in the Strategic Case For HS2 report, which also detailed the many benefits of approving the plan and the significant pressures on the existing railway network if it was thrown out.

The reduction of the BCR is sure to be seized on by opponents of the scheme, including councils and residents’ groups in Tory heartlands through which the London-to-Birmingham phase of the line will pass once completed in 2026.

While Labour questions the cost of the project, the government and unions remain passionately supportive, with Transport Secretary Patrick McLoughlin saying that the scheme was “a long-term solution to a long-term problem.”

He added that without it the West Coast, East Coast and Midland main lines were likely to be “overwhelmed” and that completing the scheme would “transform inter-city travel, radically improve commuter services into London and other major cities and increase the amount of rail freight.”

Train drivers’ union Aslef said the current plans should be bolder and were overdue. 

“We would like to see HS2 running the length of Britain. Our vision is not shared but we welcome all investment. It is not enough, we need more long-term thinking, and we should have started years ago,” said Aslef general secretary Mick Whelan.

Meanwhile, transport union RMT said that without the upgrade Britain will “slide further behind the rest of Europe on rail modernisation.”