Boris Johnson will unveil a new virus alert system, similar to how the government currently communicates terror threat levels, in Sunday’s address to Britain that will set out some of the conditions for removing restrictions on the economy.

The five-level alerts range from one (green), which would enable life to return to normal, to five (red). England is on the verge of improving from level four to three, which indicates that the Covid-19 infection rate isn’t increasing significantly, Johnson is expected to say. A new mantra is also set to make its debut: “Stay alert, control the virus and save lives.”

“We will take the decisions that continue to save lives and protect the NHS, proceeding with maximum caution and maximum conditionality,” Johnson will say, according to a government official.

Johnson will urge workers and businesses to stay alert by following social distancing rules, potentially angering some backbenchers in his Conservative Party who want a swift end to the lockdown that has scarred the economy. A person familiar with the matter earlier suggested that rules may not change significantly until June, when small shops may be allowed to reopen.

The government will also announce long-awaited measures to deal with people flying or sailing into the country. Travellers, including Britons returning from abroad, will be told to self-isolate for 14 days as part of the government’s plans to try to avoid a second peak of the coronavirus.

The new rules, set to kick in next month, will force travellers to declare where they plan to stay while they’re in isolation. Anyone found breaking the rules faces a fine of as much as 1,000 pounds or possible deportation.

The measures are likely to heighten criticism that the government was slow to react to contain the pandemic, which has now killed more than 31,000 people in the U.K. — the highest toll in Europe.

Even in late March, an influential group of cross-party lawmakers asked the government why it hadn’t set out plans to better monitor people arriving at the nation’s airports and ports.

For the world’s beleaguered airline industry, the measures give travelers yet another reason to think twice before taking to the skies and will probably end Britons’ plans of holidaying abroad this summer.

British Airways, which has London’s Heathrow airport — Europe’s busiest with some 80.9 million passengers a year — as its home, is cutting 12,000 jobs, and Virgin Atlantic Airways Ltd. is seeking to stave off collapse.