THE chairman of the company behind HS2 admits the biggest concern over the project is a lack of time.
Sir David Higgins insisted this week work on the scheme will begin on time in 2017.
But the HS2 Ltd chairman admitted when questioned by members of the Transport Select Committee on Tuesday that "an enormous amount of work needs to be done" to ensure work can get underway as planned in three years' time.
Last week Sir David recommended the first phase of the line should terminate at Crewe instead of Birmingham - but in this week's meeting said there needed to be greater certainty about the project before construction work could start.
Plans for a redeveloped Euston station to join HS2 with the existing HS1 line to Europe and a spur line to Heathrow Airport at Old Oak Common have also previously been mooted, but no final decision had yet been made.
Ahead of the second reading of the scheme's Hybrid Bill - which Sir David told committee members would take place in the Commons on Tuesday, April 29 - he said he is content the £42billion budget can successfully deliver the project.
He said: "While it's tight, it's achievable. The main risk is time, and change of scope. It's important to make decisions such as Euston and Old Oak Common now, and certainly before we start construction.
"Time is money. It's not only money in terms of the cost of how long it takes to build something, but also money on inflation.
"If we keep to the current schedule...an enormous amount of work needs to be done."
Sir David added: "We could well lose a lot of time, or gain a lot of conditions which could add cost to the overall project. The legislative process is part of the risk."