A barge docked off the Dorset coast will hold around 500 asylum seekers for at least 18 months, the Home Office has confirmed despite facing legal threats from local Tories.

“Basic and functional accommodation” will be provided on the three-storey Bibby Stockholm along with 24/7 security under Government plans to reduce the costly reliance on hotels.

The Home Office has not set out the cost of the lease agreement for the vessel that will be docked at Portland Port, but estimates have put the overall price tag of the charter and the berthing at more than £20,000 a day.

Tory-run Dorset Council and local Conservative MP Richard Drax are considering launching legal action to prevent the barge being docked near the seaside resort of Weymouth.

Human rights organisations criticised the “cruelty” of “confining” hundreds of vulnerable people on a barge, as they blamed the Government for creating the backlog of asylum claims.

Immigration minister Robert Jenrick said: “We will not elevate the interests of illegal migrants over the British people we are elected to serve.

“We have to use alternative accommodation options, as our European neighbours are doing – including the use of barges and ferries – to save the British taxpayer money and to prevent the UK becoming a magnet for asylum shoppers in Europe.

“All accommodation will meet our legal obligations and we will work closely with the local community to address their concerns, including through financial support.”

The Home Office said people will be moved onto the barge, which is currently in Italy, in the “coming months”, while other ports and further vessels will be “announced in due course”.

The vessel will provide “basic and functional accommodation, and healthcare provision, catering facilities” to 500 single adult males while their claims are being processed.

Around-the-clock security will be in place on board “to minimise the disruption to local communities”, the Home Office added.

Around 51,000 asylum seekers are currently in temporary accommodation such as hotels as the Government tackles a backlog of claims.

Confining hundreds of people in isolation on a barge is just more of the political theatre that the Government has created to obscure its gross mismanagement of the asylum system

Dorset Council said Home Secretary Suella Braverman is yet to address a “number of questions”.

“We still have serious reservations about the appropriateness of Portland Port in this scenario and we remain opposed to the proposals,” the local authority said in a statement.

Amnesty International UK’s refugee and migrant rights director Steve Valdez-Symonds said all plans to hold migrants in former military barracks and boats should be “abandoned”.

“Confining hundreds of people in isolation on a barge is just more of the political theatre that the Government has created to obscure its gross mismanagement of the asylum system,” he said.

“Instead of more ministerial cruelty, we need sweeping asylum reforms, with an emphasis on deciding claims fairly and efficiently, acting on those decisions, eliminating wasteful repeat reconsiderations of decisions that people are entitled to asylum, and making a real effort to reduce huge backlogs and unreasonable Home Office workloads.”

Liverpool-based operator Bibby Marine Limited says the 93-metre-long barge has been refurbished since it was described as an “oppressive environment” used for asylum seekers in the Netherland.

Prime Minister Rishi Sunak defended the use of the barge, insisting it would “save the taxpayer money” while being a “fair” approach to tackling small boat crossings.

On a local election campaign visit to Peterborough, he said: “We are spending, as a country, £6 million a day housing illegal asylum seekers in hotels – that can’t be right.”

Labour shadow home secretary Yvette Cooper said the barge was the result of the Conservatives’ “total failure to clear the asylum backlog”.

“This barge is in addition to hotels, not instead of them and is still more than twice as expensive as normal asylum accommodation. It will house just 0.3% of the current Tory backlog which has sky-rocketed and is continuing to grow under the Conservatives,” she added.

Portland Port chief executive Bill Reeves said: “We encourage everyone in the community to approach this with an open mind and help us show other areas just how successful this type of initiative can be, both for the migrants and the local community.

“There will be close co-operation with local agencies, including the health and emergency services during the preparations for the vessel’s arrival and its operation. We will also work closely with local community and voluntary groups.”

But Refugee Council chief executive Enver Solomon said the “completely inadequate” accommodation will not provide the “respect, dignity and support” the asylum seekers deserve.

“These muddled plans are a consequence of an asylum system that is beset with chronic delays and a record decision-making backlog, as is the significant increase in the use of hotels in recent years,” he added.

“There would be no need to use barges and former military bases if cases were dealt with in a timely and efficient manner.”

Mr Drax, the South Dorset MP, has said the barge was being “dumped on our door” and has been demanding Ms Braverman scrap the plans.

“Every option’s being looked at including legal action,” he told the PA news agency ahead of the announcement, as he sought to get the project “consigned to the dustbin”.

He cited his concerns as being a hit to local tourism and the strain on policing the port.

The Home Affairs Committee chairwoman Labour MP Dame Diana Johnson told BBC Radio 4’s PM programme that accommodating around 500 people on a barge “is a drop in the ocean, if you don’t mind me using that expression”.