The government and HS2 bosses knew the new high speed railway project was over budget and behind schedule three years ago, it has been reported.
The BBC said documents written in 2016 before MPs signed off on the first phase show that Parliament and the public were not given the facts about the true cost.
Earlier this month, the government said it will review the entire project and will give a “go or no-go” decision by the end of this year.
In May 2016, then chancellor George Osborne received a letter from transport secretary Patrick McLoughlin admitting that the first phase of the railway was already £1 billion over budget.
The budget for this phase of HS2, linking London to Birmingham, is £24 billion.
The letter to Mr Osborne revealed that a one-year deal to the opening of phase one was already being considered to “bring cost savings”.
It also showed that HS2 had failed a Review Point One, which one former HS2 director told the BBC “was like saying it wasn’t fit for purpose”.
The £1 billion overspend was considered at the time to be “a very conservative estimate”, a former director said.
"Internally, the teams knew it was a lot higher than that," he added.
The BBC also reported that unrealistic estimates were made in an “ad-hoc manner” for how much the land and property needed to build the railway would cost.
The estimate for the cost of land was £2.8 billion but in reality it was £4.8 billion, the BBC reported.
The Department for Transport said: "Like all major, complex projects delivery plans evolve over time.
"We regularly keep Parliament and members of the public updated on the progress of the project.”
Phase one of HS2 would be followed by a second phase linking Birmingham to Manchester and Leeds.
The first phase was due to open at the end of 2026, with phase two coming by 2032/33.
The total cost of the railway was supposed to be £55.7 billion.
Last month, a leaked letter indicated that HS2 could be up to £30 billion over budget.