The prime minister has said the Covid-19 pandemic "is not over" and said mask use would continue to be advised in some indoor spaces - even after legal restrictions end on July 19.

At a Downing Street press conference, Boris Johnson said: "It is absolutely vital that we proceed now with caution and I cannot say this powerfully or emphatically enough: this pandemic is not over.

He added: "This disease, coronavirus, continues to carry risks for you and your family. We cannot simply revert instantly from Monday July 19 to life as it was before Covid.

"We will stick to our plan to lift legal restrictions and to lift social distancing, but we expect and recommend that people wear a face covering in crowded and enclosed spaces where you come into contact with those you don't normally meet, such as on public transport."

The prime minister said a "gradual return to work over the summer" is expected rather than a rush back to the office en masse.

"We're removing the government instruction to work from home where you can but we don't expect that the whole country will return to their desk as one from Monday."

When it comes to nightclubs and other venues with crowds, Mr Johnson said those businesses should use vaccine passports for entry "as a matter of social responsibility".

He said: "As a matter of social responsibility we're urging nightclubs and other venues with large crowds to make use of the NHS Covid Pass which shows proof of vaccination, recent negative test or natural immunity, as a means of entry."

The prime minister emphasised that there is "no obvious date" for ending restrictions as hospital admissions and deaths from coronavirus will rise.

"We've come to a stage in the pandemic when there is no easy answer and no obvious date for unlocking," he said.

Mr Johnson explained that in his view, July 19 is the right time to ease coronavirus restrictions because of the "natural firebreak" of the school holidays.

"We also know if we were to now delay this fourth step, for instance to September, or later, then we would be reopening as the weather gets colder and as the virus acquires a greater natural advantage and when schools are back,” he said.

"We think now is the right moment to proceed when we have the natural firebreak of the school holidays in the next few days."

England's chief medical officer professor Chris Whitty said that going slowly through the next step was "essential" to reduce the impact of the "exit wave".

He said there was wide agreement between the scientific community that the four-week delay to the final stage of the road map was important to reduce the number of people admitted to hospital and deaths.

"Secondly, there is extremely wide agreement that whenever we go through the next step, there is going to be what's called an exit wave - there will be a wave associated with that," he said.

"And that the slower we take it, the fewer people will have Covid, the smaller the peak will be, and the smaller the number of people who go into hospital and die.

"So, going very slowly through this step is really essential, and this again is the overwhelming view of the scientific people who have looked at this and of the medical profession.”

The government's chief scientific adviser Sir Patrick Vallance said vaccinations are reducing the overall chances of someone transmitting the virus by about 75%.

Sir Patrick told the conference that people who had been vaccinated were both less likely to catch the virus and less likely to pass it on.

He said there was "no doubt" the country was in a third wave of the virus that would lead to an increase in the number of hospital admissions.

"If behaviour returns immediately to pre-pandemic levels that will be a very, very big rise. If we go slowly and cautiously, it will be less of a rise," he said.

On the question of whether restrictions will be eased irreversibly, the prime minister said: "I hope that the road map is irreversible but in order to have that it has also got to be a cautious approach, that's why we waited those extra weeks to get seven million more jabs into people's arms.

"We will proceed on Monday the 19th but what people need to remember is that this pandemic is not over. If we're cautious and everybody gets vaccinated, then, yes, we can make steady progress."

Professor Whitty said there was "no clear evidence" that having a further delay to the next step of the road map would make a difference.

"There is less agreement about what is an ideal date to make a move and the reason for that is there is no such thing as an ideal date - all the possible dates have downsides," he said.