LONDON - Britain is investigating the ballooning cost of building a high-speed railway between London and the north of England, the transport secretary said on Monday, after a newspaper report alleged that managers had covered up multi-billion pound budget overruns.

HS2 (High Speed 2), the publicly funded company building the line, denied the paper's claims.

The railway is aimed at helping "level up" economic growth in Britain, but it has been beset by delays and soaring costs and the UK government this month axed part of the planned line, to Manchester.

A report in the Sunday Times, citing former HS2 employees Andrew Bruce and Doug Thornton, said staff were told to keep budget estimates artificially low so lawmakers were not aware of the true cost of the project when they voted to approve construction.

Bruce, former head of planning in HS2's property department, told the newspaper: "This was a fraud against the British people."

HS2 said the allegations were "simply untrue".

"The claims made by Mr Bruce and Mr Thornton, which have been covered by the media on multiple occasions, were put under intense scrutiny by the National Audit Office," an HS2 spokesperson said.

"Its report published five years ago found nothing untoward. All land and property costs are in line with the published budget."

HS2 said it could not comment on further allegations by another ex-employee, Stephen Cresswell, that cost figures were manipulated because the claims were under investigation.

"Mr Cresswell has provided limited information directly to HS2 Ltd and we cannot comment further on the additional claims made directly to the Sunday Times," the spokesperson said.

"Putting allegations in the public domain prior to the conclusion of the investigation may compromise the investigation."

Transport Secretary Mark Harper told Times Radio a number of the allegations had been investigated and were deemed to be unfounded, but some were being taken very seriously and were being investigated at the moment.

Harper said he had strengthened the board of HS2 and had made clear that the focus was not only on delivering phase one of the project but also on hitting cost targets.

Prime Minister Rishi Sunak axed the section of the line to Manchester earlier this month, saying the 36 billion pounds ($43.8 billion) saved would be better spent on other transport projects.

The line will now only go to Birmingham, around half of its original length, but will still cost in excess of 50 billion pounds.

The government said even with the cuts, the benefits could be as low as 80 pence for every pound spent. ($1 = 0.8222 pounds)

(Reporting by Paul Sandle, editing by Deborah Kyvrikosaios)