The government and HS2 knew that the new high speed railway was over budget and was probably behind schedule years ago, documents seen by the BBC show.
Crucially, the documents were written in 2016, before MPs had signed-off the first phase of the project.
It is evidence that both the public and Parliament were not given the full picture about the true cost.
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,” the DfT added.
HS2 Ltd is a public company, set-up to build a new high-speed line linking London, Birmingham, Manchester and Leeds. It is funded by the taxpayer.
The line was due to be built in two phases, beginning with a new railway linking London and the West Midlands.
This would be followed by a second phase taking services from Birmingham to Manchester and Leeds.
Phase one of the development was due to open at the end of 2026, with the second phase scheduled for completion by 2032-33.
In total, the railway was supposed to cost £55.7bn.
Earlier this month, the government said it planned to review the costs and benefits of the rail project, with a “go or no-go” decision by the end of the year.
But until recently, ministers and bosses at HS2 have insisted everything was on track.
Updates from the Cannock Chase Radio News Desk via BBC Stoke and Staffordshire