The failings surrounding the HS2 rail project are partly due to a focus on achieving the highest possible speeds and political pressure for progress on the scheme, a review is expected to find.
The scheme has been undergoing a "reset", and in March Transport Secretary Heidi Alexander asked HS2 bosses to look at lowering top speeds to save money.
This latest report, set to be published this week, is authored by former National Security Adviser Sir Stephen Lovegrove and has considered the implications for the civil service and public sector.
It is expected to agree with a previous review that HS2's so-called "original sins" included changing political priorities and ballooning costs.
The review is also expected to highlight the "gold-plating" of the high-speed concept, "resulting in [a] bespoke and highly engineered design", which also added to the overall costs.
In the coming days, Alexander is expected to confirm that trains won't start running by the current target date of 2033 and will also provide an updated price tag for the project.
It has been widely expected that costs will exceed £100bn.
HS2's main purpose was to increase capacity on the rail network but it has suffered rising costs and delays.
Under the initial plans, first confirmed in 2012, the rail line would have run from London to Birmingham, and then on two separate lines to Leeds and Manchester.
However, in 2021, the government said it was cancelling the eastern leg going to Leeds. Two years later, the section between Manchester and Birmingham was also ditched.
In June 2025, Alexander said that after "a litany of failure" she was "drawing a line in the sand" and the government would get HS2 delivered.
Mark Wild, chief executive of the project's delivery company HS2 Ltd, was tasked with carrying out a comprehensive "reset".
The chair of the Transport Committee, Ruth Cadbury, said the problem with HS2 was "not just the speed, but also that spades were in the ground before the project had been fully designed, permits granted and so on".
"It broke the mantra of major projects, which is 'plan slow and build fast'," she told the BBC's Today programme.
Cadbury said that from the start "there was such an urgency from politicians of the time to get on with it" that the full specification of the project and all the risks associated with it had not been worked out.
Earlier this year the transport secretary said she was "determined to explore every opportunity" to "bring down costs and delivery timetables" including reducing the top speeds of trains on the line.
HS2 had been designed to allow trains to run up to 360 km/h (224 mph), which would have made the line faster than any other conventional railway in the world.
Most high-speed trains in the UK run at around 200km/h (125mph), while HS1, the Channel Tunnel Rail Link, reaches speeds of up to 300 km/h.
Cadbury said the slower expected speeds for HS2 would still be similar to high-speed lines in other countries, adding the most important thing was "to create new capacity that is so desperately needed between London and the north of England, and eventually to Scotland".
Although it will be years before the railway opens, HS2 is in its peak construction phase.
A number of key structures have been completed, for example the 10-mile tunnel under the Chilterns, and the Colne Valley viaduct.
As part of efforts to get the project back on track, HS2 Ltd has previously said it would slow or pause work such as the line towards Handsacre, so it could focus spend on areas which had fallen behind - notably the central section across Buckinghamshire, Oxfordshire and Northamptonshire.
Shadow transport secretary Richard Holden said: "Whilst Labour talk about cost, you won't hear them admit they handed their union paymasters a 15% pay rise, costing the taxpayer £135m in the first year alone, or the fact that industry leaders are warning that their nationalisation plans will drive up costs by £10bn."
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