Two more barges will be used to house asylum seekers as part of continued efforts to reduce Channel crossings, Rishi Sunak has announced.
The prime minister declined to say where they will be moored, but said they will have the capacity for an extra one thousand migrants who enter the UK illegally.
Speaking from Kent, he also announced that another ship for 500 asylum seekers, which the government acquired in May, will arrive in Portland off the coast of Dorset within the next two weeks.
This was met with a furious reaction from the local Conservative MP Richard Drax, who claimed it will be "nothing more than a quasi-prison".
He told LBC: "They've got £9 a week to spend - which isn't much money - what happens if they disappear? None of these questions have been answered."
Charities and opposition MPs also condemned the expansion of the plan and urged the government to concentrate on reducing the asylum seeker backlog rather than putting out "cynical spin".
But Mr Sunak said Channel crossings were down by a fifth compared to last year and "our plan (to stop the boats) is starting to work".
He said there was more to be done, adding: "To reduce pressures on local communities we will also house people on ships. The first will arrive in Portland in the next fortnight and we've secured another two today."
On top of this measure, Mr Sunak said thousands of extra spaces for migrants had been found in hotels by making people share rooms.
The prime minister insisted this was "more than fair" following protests outside accommodation in Pimlico, London.
He said: "If you're coming here illegally claiming sanctuary from death, torture or persecution, then you should be willing to share a taxpayer funded hotel room in central London."
The Home Office later said the move will save £250 million a year and could reduce the need to source an additional 90 hotels.
But Yvette Cooper, Labour's shadow home secretary, said this measure is "not new".
She told BBC Radio 4's World At One programme: "The real problem is that they're still increasing the number of hotels, increasing the number of hotel rooms that they're using in the first place, and that reflects this failure to just take basic decisions to actually clear the backlog, to make decisions and get the system properly working rather than this chaos."
Crossings 'down by a fifth'
Some 172,758 people were waiting for an initial decision on asylum applications at the end of March, up 57% from a year earlier and the highest figure since comparable records began in 2010, according to Home Office figures.
Mr Sunak said numbers published on Monday show the backlog is down by more than 17,000 and insisted the government was "on track" to clear it by the end of the year.
He also said that Channel crossings were down by 20% and his returns deal with Albania had led to 1,800 people being sent back, and was having a deterrent effect.
Mr Sunak said: "Before I launched my plan in December, the number entering the UK illegally in small boats had more than quadrupled in two years. Some said this problem was insoluble, or just a fact of 21st century life.
"They'd lost faith in politicians to put in the hard yards to do something about it. And of course, we still have a long way to go. But in the five months since I launched the plan, crossings are now down 20% compared to last year."
He went onto defend the inclusion of children in new detention rules, claiming that to exempt them would create an "incentive" for smugglers to put more young people on boats.
And he said preparations are being put in place so that once legal challenges are complete "we have more detention capacity to hold those who arrive illegally, enough court capacity to process their cases and the planes to remove them".
"With grit and determination, the government can fix this and we are using every tool at our disposal," he said.
The speech comes after polling found the majority of the British public think the prime minister is failing on his five priorities, which include stopping small boat crossings.
Mr Sunak has staked his premiership on reducing illegal immigration and has previously announced plans to house asylum seekers in former military barracks to reduce reliance on hotels, which the government says is costing the tax payer £6m a day.
The Home Office said sites at Wethersfield and Scampton - which have faced objection from local Tories - will open this summer and house 3,700 people.
Labour leader Sir Keir Starmer said the promise of further measures to tackle the crisis was "like Groundhog Day" and the government should focus on securing more returns agreements.
He told reporters in Somerset: "We need to stop the boats. We're clear we don't want anyone making that dangerous journey.
"But all we've had from the government is policies that aren't working, then the reannouncement of the same policy, with a self-congratulatory pat on the back. It feels like groundhog day and it's costing the taxpayer a fortune."
Meanwhile the Liberal Democrats dismissed Mr Sunak's speech as "cynical spin" to cover up the fact they "have broken our asylum system".
Charities also condemned the plan.
Amnesty International UK said the barges plan was "potentially unlawful" and a "terrible idea" designed to distract the public from the government's failure to tackle the asylum claims backlog.
The Refugee Council branded Mr Sunak's rhetoric on migrant crossings "misleading, wrong and harmful" and said his policies fail to treat people fleeing their home country with dignity and humanity.