The cost of building a bat protection tunnel structure on a stretch of the HS2 route in Buckinghamshire has risen to £100m.

The initial package for the arched structure spanning four tracks – two HS2 lines as well as two East West Rail lines – was originally budgeted at £40m when procurement started three years ago.

But HS2 chairman Sir John Thompson confirmed the building he referred to as a “shed” will now cost £100m.

Sir John told a rail industry conference this week: “This shed, you’re not going to believe this, cost more than £100m to protect the bats in this wood.”

The tunnel is designed to protect a colony of endangered Bechstein bats located in the ancient Sheephouse Wood, designated as a site of Special Scientific Interest, which lies next to the route.

The Sheephouse Wood Bat Mitigation Structure will cover a 900m stretch of the railway that lies in the flight path of the rare bat species.

Construction at the site is being overseen by main HS2 contractor EKFB.