U.K. Prime Minister Boris Johnson on Feb. 11 finally approved construction of the country’s controversial high-speed railroad, HS2, after huge cost hikes put the program in doubt.

Construction of phase one, from London to Birmingham, will start in April, said Johnson, while legislation will be resumed to approve the next, short phase, from Birmingham to Crewe. Main contracts for phase one were conditionally awarded in 2017.

The government’s decision follows previous recommendations to proceed made in a report led by former HS2 Chairman Douglas Oakervee. However the report also sets out large cost hikes, which are themselves considered to be underestimates by the National Audit Office, and others. While the government will scrutinize the cost of phase one, there would be no “detailed redesign,” added Johnson.

At 2019 prices, phase one will cost up to $56 billion, which is $16.3 billion above budget. Cost of the total program, including extensions to Manchester and Leeds, at $114 billion, would overshoot the previous estimate by $33 billion.

On the other side, cancelling the project now would cost around $11.7 billion and would have “significant detrimental consequences for the supply chain and the fragile UK construction industry,” notes the Oakervee report.

With significant involvement in the program, Atkins, the infrastructure arm of SNC-Lavalin, welcomed the decision. Lizi Stewart, the firm’s transportation managing director, urged the government “to move ahead with purpose to ensure that Phase 2 isn’t compromised.”

However, Stop HS2 Campaign Manager Joe Rukin called the decision “Boris’ big blunder” and claimed that the project threatens almost 700 habitat sites and over 100 ancient woodlands.