Plans to moor a large vessel beside London Bridge have run into an unexpected complication – a disused tube tunnel buried beneath the Thames.

The proposal would see a floating Livery Hall for the Honourable Company of Master Mariners (HCMM) moored on the north bank of the river beside Swan Lane. Ordinarily, a boat floating on the river would have little impact on infrastructure deep below. However, this stretch of the Thames sits directly above one of London’s forgotten railway tunnels.

The tunnel was built in 1890 by the City and South London Railway (C&SLR), linking Stockwell with its original terminus at King William Street, close to Monument. The line was notorious for the steep gradient leading into the station. When the railway was extended northwards to Angel, a new route under the Thames was constructed, and the original tunnel was abandoned.

Although trains have not used it for more than a century, the tunnel still exists. Indeed, access to the former King William Street station proved invaluable during the recent upgrade works at Bank station.

You might wonder how a boat floating in the Thames would affect tunnels deep under the river bed.

For this, you can blame some dolphins.

No, not the chattering sea mammals, but the structures that support riverside piers and that moats can moor up against. They are called dolphins, and although there are two in the river where the Livery Hall boat could be moored, some construction work might be needed to strengthen them.

Those structures sit directly above the abandoned railway tunnels, meaning any works to reinforce them would need to account for the tunnels.

If approved, the boat would be a replacement City Livery Hall for the Honourable Company of Master Mariners (HCMM), which used to be at HQS Wellington until 2023.

The scheme has attracted support from a former Admiral and several other Livery Companies, but it has also generated numerous objections. Among those raising concerns are the Port of London Authority, the River Residents Group and, to a lesser extent, the Greater London Authority. While all have expressed support for using the pier, they question whether this particular proposal is appropriate.

There are also suggestions that the new Livery Hall boat would be bigger than the Oceandiva, which previously tried to moor up at the piers and was refused permission.

The planning application is now awaiting a decision from the City of London Corporation.