The Fire Brigades Union (FBU) has sent a “pre-action protocol letter” to home secretary Suella Braverman outlining its concerns over safety aboard the vessel moored in Dorset’s Portland Port in southwest England.

The union previously branded the giant barge, initially designed for about 200 people but modified to house 500 migrants, a “potential death trap”.

The first asylum seekers placed on board the Bibby Stockholm earlier this month were removed days later after tests revealed Legionella – a bacteria that can cause the potentially fatal Legionnaires’ disease.

The FBU is demanding a response to its legal letter by Thursday.

General-secretary Matt Wrack said the FBU has been sounding the alarm about the Bibby Stockholm for weeks. The FBU has raised concerns about access to fire exits and possible overcrowding on the vessel.

Wrack said: “It is disgraceful that the home secretary is not even willing to meet us to discuss these concerns. Throughout this episode, the government has displayed a lack of transparency and a callous disregard for the safety of both firefighters and those who are due to be housed on the barge.”

“Fires do not discriminate based on immigration status, and neither can fire safety regulations.”

The Bibby Stockholm is one of several sites, which also include former military bases, ministers want to use to house migrants as they await asylum decisions in an effort to cut the cost of putting them up in hotels and deter entries into the UK via unauthorised means.

Home office data this week showed Channel crossings topped 19,000 for the year so far, despite Prime Minister Rishi Sunak’s pledge that he will “stop the boats”.

The home office has been approached for comment.