With British Prime Minister Boris Johnson in intensive care in hospital with coronavirus, Foreign Secretary Dominic Raab has stepped in -- but he will not have the same power.

Here are some of the key questions about who is now running Britain during this crisis.

Who is in charge?

Before he was admitted to intensive care on Monday evening, Johnson asked Raab, whose other job title is First Secretary of State, "to deputise for him where necessary", a Downing Street spokesman said.

Raab had already chaired the government's daily coronavirus briefing on Monday after Johnson was first admitted to hospital for tests on Sunday night. Raab took charge again on Tuesday morning.

He is expected to receive Johnson's daily briefing papers in his 'red box' for official dispatches, according to the Institute for Government (IfG) think tank. He will also coordinate the work of other ministers who chair cabinet sub-committees dealing with specific areas of coronavirus, such as the healthcare response and efforts to support businesses.

Will Britain's response to coronavirus change?

Raab challenged Johnson for the leadership of their Conservative party last year, but on Monday evening insisted he would follow his boss's plan to tackle COVID-19.

"The focus of the government will continue to be on making sure that the prime minister's direction... will be taken forward," he said.

Britain has been in lockdown since March 23, with the public told to stay at home wherever possible and most shops and services closed. However, these measures are up for review next week -- a big decision.

Cabinet minister Michael Gove said the decision would not be delayed but would be taken "collectively as a cabinet", with Raab having the final say if Johnson is not able to make the call.

What about national security?

Tobias Ellwood, chairman of the House of Commons defence committee -- which acts as a watchdog -- warned Britain must be ready for "adversaries attempting to exploit any perceived weakness" while Johnson is in hospital.

"It is important to have 100 percent clarity as to where responsibility for UK national security decisions now lies," he tweeted.

Gove added: "It's Dominic as foreign secretary who's in charge".

Could Raab overreach?

There is no constitutional role of acting or deputy prime minister.

Britain is formally governed by the cabinet, and even the prime minister, while having certain powers, can only govern with the support of cabinet ministers. "Raab has to be careful... of not overstepping the bounds of the authority he's been given," said IfG director Bronwen Maddox.

She suggested cabinet colleagues would "rally round" given the extraordinary circumstances, but if reported tensions over strategy continued, Raab might find things difficult. "It's harder for someone deputising for the prime minister really to exert authority over divisions if those appear," she told BBC radio.

How has this worked before?

Prime ministers often delegate certain tasks to a cabinet colleague, for example asking them to chair meetings while they are on holiday. However, they are kept informed and remain in charge.

During the Cold War, prime ministers appointed "nuclear deputies" to decide what to do in the event of the prime minister being incapacitated or out of contact.

But the rules are fluid. The IfG highlighted how, in 1953, then prime minister Winston Churchill reportedly suffered a stroke.

The most obvious replacement, foreign minister Anthony Eden, was himself in hospital undergoing surgery, and in the end the news was kept from most of the cabinet and Churchill carried on.

What happens if Johnson dies?

Although the titles of deputy prime minister or first secretary of state are used, it does not mean the office holder automatically takes over.

The IfG says it would be up to the cabinet to collectively recommend an immediate successor to Queen Elizabeth II, who would then name them to take over. "This could be done with the expectation that his or her role would be temporary, pending the election of a new party leader," it said in a briefing note.

But as there is no formal role of acting prime minister, they would stay in power "until they chose to resign or if their cabinet forced them out".