Suella Braverman is facing demands from firefighters for an urgent meeting over the safety of a giant barge meant for asylum seekers amid a deepening row over whether it is a “deathtrap”. The Fire Brigades Union (FBU) has questioned a lack of exit points, the narrowness of corridors and increased occupancy on the Bibby Stockholm in a letter sent to the home secretary on Wednesday. Originally built with 222 cabins, the barge is now expected to contain more than 500 people in Portland, Devon. The letter was sent after Grant Shapps, the energy secretary, disputed claims made by the union that the barge was a “potential deathtrap”. Plans to move people seeking asylum on to the barge have been beset by delays, with government sources now suggesting the first arrivals may not be onboard until next week at the earliest. In the letter to Braverman, Ben Selby, the FBU’s assistant general secretary, said: “We are concerned about the risks on a large floating structure used to accommodate asylum seekers in long-term housing and would expect the risk assessment to recommend substantial prevention and control measures to tackle overcrowding, access to fire exits and other safety matters on the vessel. “The FBU seeks an urgent meeting with you and your officials to discuss these matters. We have substantial expertise, including from earlier disasters such as the Grenfell Tower fire and the subsequent public inquiry. We are concerned above all to prevent another tragedy.” Speaking on Sky News, Selby said the union’s main concern was the number of people due to live on the barge, which has been adapted in the UK to include more bedrooms and bunkbeds, taking the capacity to about 550. “That then raises significant fire safety concerns for us, and also concerns that, if a fire was to break out on the Bibby, could firefighters make the adequate rescues and access where necessary?” he said. In a further development, government sources have claimed that the union’s criticisms are part of a cynical plan by Labour and the unions to keep asylum seekers in hotels – a claim the union vehemently denies. An insider said that the FBU is affiliated with Labour, and said: “Labour and its union backers are gleeful at the idea of housing migrants in hotels because it suits their political games. And that’s why they don’t want to stop the boats – because they’ve also calculated it’s to their gain.” Responding to the criticism, Stephen Kinnock, the shadow immigration minister, said: “Instead of getting a grip and clearing the backlog, all they offer is gimmicks and lashing out trying to blame everyone else. They’ve been in charge for 13 years – this is their chaos.” Earlier, Shapps told ITV’s 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 government is struggling to move people seeking asylum out of hotels and into alternative accommodation such as giant barges, cruise ships and disused military bases, despite repeated promises from Rishi Sunak that he will find alternatives. The use of hotels has risen in recent years after a succession of record highs in the number of small boats crossing the English Channel. The prime minister has named “stopping the boats” as one of his five priorities for office since taking over the job in October, but key planks of his policy – including sending asylum seekers to Rwanda to be processed – have been stalled. On Tuesday, Sunak rejected claims that the barge plan had been a shambles. “This is an example of me doing something different that hasn’t been tried before to help solve a serious problem,” he told LBC. The first 50 migrants were expected to arrive on board the vessel on Tuesday but Home Office sources said that late representation from the Health and Safety Executive (HSE) to check working practices for port staff delayed the plans – and not fire safety concerns. The HSE said it had raised concerns about hi-vis clothing and separating pedestrians from vehicles, but saw no reason for its recommendations to cause the holdup.