Ministers are "considering very seriously" building a controversial new high-speed rail line between London and Birmingham, the BBC understands.
A source said a Network Rail review of the two main alternatives favoured by opponents concluded they could not "generate the capacity" needed.
The government is due to make its final decision on the £17bn HS2 scheme next week.
The 100-mile connection would be built between 2016 and 2026.
It aims to cut the journey time between London and Birmingham to 49 minutes. It would mark the first phase of HS2, with extensions further north later.
A Y-shaped section taking branches to Manchester, Leeds and possibly further north could be finished by 2033.
The entire cost of the project is expected to be £32bn.
A government source told the BBC: "Groups opposed to high speed rail claim there are alternative packages of railway improvements that can bring similar benefits without making any sacrifices.
"This independent Network Rail report shows that the main alternatives cited by opponents cannot in fact generate the capacity and connectivity boost that a new high-speed rail line could deliver.
"This is why the government is in the process of considering very seriously the question of building such a line."
The project - introduced by Labour and continued by the coalition government - has proved highly controversial.
Opponents say the planned route crosses an area of outstanding natural beauty and it will damage the environment. It also passes through Conservative heartlands and some Tory MPs have strongly objected to the proposal.
Critics have argued that overcrowding can be eased by improving the existing line, running longer trains and having fewer first-class carriages.
This latest review by Network Rail looked at two alternative schemes which suggest a series of improvements to the existing West Coast Main Line (WCML).
It found that neither would provide enough capacity to meet the predicted passenger demand and both would result in long delays during work on the infrastructure.
The report also found that while cost estimates for the two alternatives were "realistic", other factors such as remodelling work at London's Euston station had not been taken into account and the cost of disruption had been underestimated.
It concluded they would "deliver considerably fewer benefits than a new line".
A Network Rail spokesman said: "The capacity case for a new high-speed line is clear. In just over a decade the WCML, Britain's busiest and most economically vital rail artery, will be full with no more space to accommodate the predicted growth in demand.
"Alternative schemes to HS2 have been put forward which would deliver some short-term capacity benefits, but they would come at a heavy price in terms of disruption to passengers and the wider economy."
Lucy James, from the Campaign for High Speed Rail, said: "This report is just the latest piece of evidence to show that HS2 is the only game in town when it comes to solving the capacity crisis on Britain's railways."
Penny Gaines, from the Stop HS2 campaign, said it was difficult to understand how Network Rail could claim that the alternative plans would cause too much disruption.
"A low-risk series of incremental improvements will bring more benefits to more people more quickly for less money," she said.
Under the current proposal, London's Euston station would need to be rebuilt and that would take seven or eight years, she added.