British Prime Minister Boris Johnson issued new stay-at-home advice on Sunday and warned he would proceed cautiously on lifting a nationwide coronavirus lockdown as the death toll in Britain, already the highest in Europe, continues to mount.

In a televised address to the nation at 7:00 pm (1800 GMT), Johnson is expected to extend confinement orders imposed in late March, although garden centres are set to reopen.

But in a sign of a gradual shift towards getting the country moving again, he tweeted new public advice for people to "stay alert", to replace the current slogan to "stay home, save lives".

People must stay at home "as much as possible" and limit contact with others, a more lenient message than previous rules to only go outside when necessary and to meet nobody outside your of own household.

Johnson, who spent a week in hospital with coronavirus last month, has said he will proceed with "maximum caution" in lifting the lockdown. He is expected to extend the rules on Sunday by announcing a mandatory 14-day quarantine for anyone arriving into the UK.

However, his new 'stay alert' slogan drew criticism from Scottish First Minister Nicola Sturgeon, who warned infection rate in her country remained too high to ease public advice.

"We mustn't squander progress by easing up too soon, or by sending mixed messages that result in people thinking that it's OK to ease up now," she told reporters. "Let me be very blunt about the consequences if we were to do that - people will die unnecessarily." The devolved governments in Scotland, Wales and Northern Ireland have already extended their lockdowns, and Johnson's plan will focus on England.

He will also unveil an alert system to monitor the outbreak, which will inform the use of lockdown measures at a national and local level.