Boris Johnson will decide whether to bring in tougher measures to delay the spread of coronavirus on Monday, after the number of British cases increased by a third to 278 and the number of deaths in the UK rose to three.

The prime minister is to chair his first emergency Cobra meeting on the virus since last Monday, which will look at possible measures if the UK formally moves from trying to contain the outbreak to delaying its impact.

The government’s coronavirus plan, published last week, included suggestions such as social distancing and increased home working at the milder end of the spectrum, to cancelling mass-attendance events and cutting back on essential police and fire services at the more extreme end.

The UK saw its biggest one-day increase in coronavirus cases on Sunday, with 69 new cases confirmed. A man in his 60s, who had underlying health problems, became the third patient to die after testing positive for Covid-19 in the UK. The man, who died at North Manchester general hospital, had recently returned from Italy, NHS England said.

As the government prepared emergency legislation to allow some court proceedings to be heard by phone or video, GPs warned that hospitals would have to cut back on work not related to coronavirus in order to tackle the outbreak.

Ahead of the Cobra meeting, Johnson insisted the UK was “well-prepared and will continue to make decisions to protect the public based on the latest scientific advice”. “Tackling coronavirus will require a national and international effort. I am confident the British people are ready to play their part in that,” he said. “The most valuable thing people can do is wash their hands with soap and water for 20 seconds.”

The government is considering plans to let more people volunteer in the NHS, and the health service said it had already trained about 500 additional initial call responders to help deal with increased demand for the 111 service.

The health secretary, Matt Hancock, said the government would do “everything in its power” to delay and mitigate the coronavirus threat as he set out plans contained in emergency legislation to deal with the impact of the virus.

The bill, which is likely to go through parliament by the end of the month, is expected to include measures to allow some court proceedings to be conducted via telephone or video.

The news came as 32 people who had been aboard a cruise ship in Japan were released from quarantine in Wirral, and two people who had recently returned to Wales from Italy tested positive for the illness.

The Foreign Office said it was “working intensively” with US authorities to arrange a flight for British nationals on the coronavirus-hit Grand Princess cruise ship, which is due to arrive in Oakland, California, on Monday.

A healthcare worker at University Hospital Southampton NHS foundation trust has tested positive for coronavirus. The surgical high-dependency unit, where the person worked a single nightshift on Friday, has been temporarily closed to new admissions as a result.

Prof Martin Marshall, chair of the Royal College of GPs, warned that the Covid-19 outbreak was a “significant crisis” for the health service but said that estimates that 100,000 could die were a “worst-case scenario”.

“If we are going to try to continue doing what we are doing at the health service and tackle coronavirus, it will require a significant amount of resources,” he told Sky’s Sophy Ridge on Sunday. “I expect we will cut down in terms of the other work we do.”

Marshall welcomed plans to call upon recently retired doctors to expand the workforce. He said: “I think it is a good idea as long as we do it carefully … and people are estimating that 20% of the workforce might be out of action at any one time when the crisis reaches its peak – people are talking about some time in late April for that. If that happens, then we do need to expand the workforce in whatever way we can.”

World Health Organization representative Dr Margaret Harris warned that government must ensure that supporting health workers is their “number one priority”. She said: “They need backup, they need other people to come and do the shifts. If they’re working massively, they are tremendously at risk.”

Asked if army field hospitals could play a role in the response to the coronavirus outbreak, Harris said: “Certainly the army has great experience of putting up field hospitals. I’ve worked on Ebola and I’ve seen what the British army can do. It’s quite incredible.

“So this is the sort of planning you should be thinking about. Can you set up a field hospital? Where do you set it up? What equipment have you got and what staff have you got and how can you protect everybody working in those conditions?”

Rishi Sunak, the chancellor, has promised to give the NHS “whatever it needs” to tackle the coronavirus crisis, as he looks at loosening the fiscal rules to allow for more borrowing and spending ahead of this week’s budget.

The Scottish first minister and SNP leader, Nicola Sturgeon, told Ridge it was “pretty much inevitable that we will need additional resources” in the NHS to cope with coronavirus. A total of 18 people have tested positive for the virus in Scotland.

Government figures released on Sunday morning revealed 64 new coronavirus cases in the UK, with more than 23,513 people tested as of 9am. A further five cases were reported as Sunday wore on. Three people had died as of Sunday night.

Prof Tom Solomon at the Walton Centre NHS foundation trust said that this increase was “not really unexpected” because “it is likely that the number of cases in this country is going to double every few days”.

On Sunday, NHS England announced that 32 people have been given an all-clear for coronavirus after being held in isolation in Wirral after returning from the Diamond Princess cruise liner in Japan. The group – made up of 30 Britons and two Irish people – had been held in quarantine since last month after the ship became a breeding ground for the virus.

Seven people, including one UK citizen, who had been aboard the ship have died so far. Two further patients in Wales also tested positive for coronavirus after returning from northern Italy.