The Bibby Stockholm is a barge, currently docked off the coast of Dorset, which the government hopes will soon be home to 500 people who entered the UK illegally.
The Fire Brigades Union (FBU) hit out at plans it says the government has to exempt asylum accommodation from requirements for an HMO license, which means they may escape local council safety checks.
It said it is “concerned about a potential lack of ingress and exit points, narrow corridors and doorways, and increased occupancy”.
Assistant general secretary Ben Selby said: “The government has already scrapped vital fire safety measures for asylum seeker accommodation. Now, it wants to put more than 500 people onto an off-shore barge designed to hold around 200.
“This is a cruel and reckless approach to the welfare of asylum seekers, and puts the safety of firefighters at risk.”
He added: “Attacking asylum seekers will not build a single house, train a single nurse or pay anyone a decent wage – it is a technique for dividing working people against each other.”
In his letter, Mr Selby warned that: “firefighting operations on vessels such as the Bibby Stockholm provide significant challenges and require specialist training and safe systems of work.
“The diminished safety provisions only exacerbate our operational concerns”.
The FBU said it was requesting a meeting with Ms Braverman “to prevent another tragedy” like the Grenfell Tower disaster.
Cabinet minister Grant Shapps sought to refute concerns raised by the FBU when speaking to ITV on Wednesday morning.
He told Good Morning Britain: “It certainly won’t be a deathtrap.
“This actual ship was previously used by Germany to house migrants, there’s no reason why it wouldn’t be absolutely safe. Ships are used to transport people all the time and there’s no inherent reason why that (not being safe) would be the case.
“That’s actually why these final safety checks are being carried out.”
The prime minister has been keen to use off-shore accommodation to house migrants instead of paying for expensive hotels but there has been intense local opposition.