The Home Office has been forced to delay moving people seeking asylum on to a controversial giant barge in order to carry out last-minute fire safety checks amid concerns the vessel has not received approval from inspectors. The Guardian understands the initial plan to move refugees on to the Bibby Stockholm in Portland, Dorset, has been moved from Tuesday to Wednesday for officials to give a final survey. It follows reports that the barge could be a “floating Grenfell” and endanger the lives of vulnerable people who have fled hardship and war as it has not received the relevant signoff. About 40 claimants currently staying in other Home Office accommodation have received transfer letters saying they will be moved to the floating 222-cabin vessel in Dorset, Whitehall sources said. One letter, seen by the Guardian, says: “Your accommodation is changing and you will be moved to the Bibby Stockholm, Portland. “You are not being detained under immigration powers and this is not detention accommodation. You are free to leave the site but we would request that you sign in and out of the site when you leave and return so we can assure your safety.” The letter, which also details “several facilities” on the barge including an “onsite nurse” and “wifi”, is signed by “The Home Office Alternative Accommodation Team”. More than 50 national organisations and campaigners, including the Refugee Council, Asylum Matters and Refugee Action, have called the government plan “cruel and inhumane”. They said the vessel was “entirely inappropriate” and would house traumatised migrants in “detention-like conditions”. One refugee charity said the letters had upset already traumatised people. Steve Smith, the chief executive of Care4Calais said: “We are now supporting terrified individuals who have received transfer letters. As expected they include people who have experienced severe trauma at sea before seeking safety in the UK.” The first group of people could be sent to the accommodation for asylum seekers in Portland, Dorset, on Wednesday, local sources said. Ministers have claimed that more than 500 will eventually be housed there. Official documents say the boat can hold 506 guests. The Times reported serious fire safety concerns about the barge, with fire service approval pending. Dorset and Wiltshire fire and rescue service said it had advised on a fire risk assessment drawn up by the vessel’s operators, Landry & Kling. The prime minister’s press secretary said on Monday: “The Bibby Stockholm is currently undergoing final preparations including fire safety checks. That’s happening this week to ensure that it complies with all the appropriate regulations. “There’s been refurbishment that’s been ongoing to ensure it complies with the marine industry safety regulations. As you’d expect, we continue to work extremely closely with the local council … to ensure the right preparations are in place before anyone boards.” Asked about reports that plans to move migrants into RAF Scampton in Lincolnshire have been delayed, the spokesperson replied: “Work is ongoing to open [the] site at Scampton and we want that work to be done as soon as possible. “I can’t get into running commentary on expected timelines but eventually the site will accommodate almost 2,000 people.” Plans to move 2,000 people into the RAF camp have reportedly been delayed until October after setbacks in conducting surveys on the 14 buildings designated for migrant accommodation. A Home Office spokesperson said: “Delivering accommodation on surplus military sites and vessels will provide cheaper and more orderly, suitable accommodation for those arriving in small boats. “The Bibby Stockholm is now undergoing final preparations to ensure it complies with all appropriate regulations before the arrival of the first asylum seekers in the coming weeks. “In addition, the Home Office is working with stakeholders on a carefully structured plan to increase the number of asylum seekers at Wethersfield in a phased approach.”