The first ever project of pioneering engineer Isambard Kingdom Brunel – London’s underwater Thames Tunnel – has this week opened to the public as a new cultural attraction and concert hall.

The 19th century tunnel – the first in the world built underwater – is now used as part of the city’s rail network. It was built in 1843 by a teenage Brunel with his father, Marc, to connect the districts of Rotherhithe and Wapping for pedestrians and carriages.

Architecture practice Tate Harmer have made the tunnel's 65ft (19.8m) deep entrance shaft in Rotherhithe newly accessible to the public by constructing a freestanding, cantilevered staircase and viewing platform down the tunnel’s shaft. The 50ft wide space in the Grand Entrance Hall, which once hosted banquets and fairs organised by Brunel, will now be used for concerts, performances and exhibitions for London’s Brunel Museum.

The project was commissioned by the museum to showcase the tunnel – described on its opening as the Eighth Wonder of the World – and the built legacy of Brunel.

Explaining the ‘ship-in-a-bottle’ design, which means the staircase is completely independent from the structure, Tate Harmer partner Jerry Tate said: “It was vital that the staircase and new entrance to the Rotherhithe shaft did not impact on its historical significance. We wanted to celebrate the raw nature of the Victorian industrial heritage while providing the public proper access for tours and performances”.

The project was funded by the Association of Independent Museums, the National Heritage Landmarks Partnership and the London Borough of Southwark. The opening coincides with the 210th anniversary of Brunel's birth.

Robert Hulse, director of the Brunel Museum, told CLAD: "From the very start of its life, Londoners took the Thames Tunnel to their hearts. In the first three months, half the population of London came for the novelty of walking under a river, but also for the novelty acts in the world’s first underwater concert hall. By 1843 this was the most popular visitor attraction in the world.

"It's a hard act to follow, but the museum will fill the underground chamber with music, theatre, opera and parties. With a grand piano and state of the art lighting rig, we aim to be classical with a modern twist – like Tate Harmer's fabulous staircase."