The first migrants have boarded the Bibby Stockholm barge amid claims several others were granted a last-minute reprieve after a series of legal challenges.

Campaigners were seen waving at coaches carrying passengers as they drove into Portland Port on Monday, while pictures showed men with luggage accompanied by staff in high-vis jackets as they walked up a gangway onto the barge moored on the Dorset coast.

Cheryl Avery, the Home Office’s director for asylum accommodation, said 15 people have been moved onto the vessel so far as part of the Government’s bid to cut the cost of hotel bills by finding other accommodation, which also includes former military bases.

Ms Avery told broadcasters: “We have had a few challenges but this is part of an ongoing structured process to bring a cohort of up to 500 people on board.”

Ms Avery said there have been “some minor legal challenges” but would not comment on the detail of them, adding accommodation is offered on a “no choice” basis.

Care4Calais said around 20 asylum seekers did not board the barge as planned because their transfers were “cancelled” after lawyers challenged the decisions.

In a series of legal letters to the Home Office, solicitors raised concerns about the suitability of the accommodation for people with disabilities, mental and physical health problems as well as those who had fled torture and persecution, according to the refugee charity.

It is not known whether the decisions to ditch the transfers is temporary and could be revisited.

It comes as Labour accused the Government of “disastrous failure” over its pledge to stop the boats after official figures confirmed the number of migrants staying in hotels has passed 50,000.

Meanwhile, it emerged one of the coaches transporting migrants to the port appeared to be operating without a valid MOT as, according to the gov.uk website, this was overdue.

Human rights campaigners condemned using the barge to house asylum seekers – a plan which has been beset by weeks of delays amid safety fears and maintenance problems.

But Home Office minister Sarah Dines said those arriving in the country via unauthorised means should have “basic but proper accommodation” and that they “can’t expect to stay in a four-star hotel”.

Care4Calais chief executive Steve Smith said: “None of the asylum seekers we are supporting have gone to the Bibby Stockholm today as legal representatives have had their transfers cancelled.

“Amongst our clients are people who are disabled, who have survived torture and modern slavery and who have had traumatic experiences at sea. To house any human being in a ‘quasi floating prison’ like the Bibby Stockholm is inhumane. To try and do so with this group of people is unbelievably cruel. Even just receiving the notices is causing them a great deal of anxiety.”

Steve Valdez-Symonds, Amnesty International UK’s refugee and migrant rights director, said: “It seems there’s nothing this Government won’t do to make people seeking asylum feel unwelcome and unsafe in this country.

“Reminiscent of the prison hulks from the Victorian era, the Bibby Stockholm is an utterly shameful way to house people who’ve fled terror, conflict and persecution.”

Earlier, while facing questions from broadcasters, Ms Dines indicated the number of migrants expected to be housed on the barge could rise rapidly to its capacity of around 500 men by the end of the week.

But Downing Street appeared to suggest she had misspoken, with Prime Minister Rishi Sunak’s official spokesman saying that while “no limit” has been set on how many people will board the barge this week, the Government’s plan is to reach the capacity “over time”, adding: “I don’t think we are aiming to do it by the weekend.”

The Home Office later clarified that the total will be reached over a longer period of time and not by the end of the week.

More than 15,000 migrants have arrived in the UK so far this year after crossing the Channel, figures show.

Ms Dines said “all possibilities” for tackling the migrant crisis are being examined, following reports that the Government is considering reviving plans to fly people who arrive by unauthorised means 4,000 miles to Ascension Island.

The developments came amid a series of announcements on efforts to curb Channel crossings – an issue Mr Sunak pledged to solve as one of his key priorities for his time in office – including plans to increase fines for employers and landlords who allow people who arrive by irregular means to work for them or live in their properties.

Rather than a “heavy-handed approach” as part of a so-called “small boats” week publicising crackdown measures, the Green Party said there should instead be a “welcome refugees week”, adding: “No barge is a suitable home for refugees … Housing them in a barge is to treat them as prisoners and is heartless.”

Some 339 migrants were detected crossing the Channel on Friday and Saturday after an eight-day hiatus amid poor weather conditions at sea, taking the provisional total for 2023 to date to 15,071. But no crossings were recorded on Sunday, according to Home Office data.