Boris Johnson has warned that step four of England’s road map in June is in jeopardy as scientists said the Indian variant of concern could be 50% more transmissible than the Kent variant.

The Prime Minister told a Downing Street briefing that if the Indian variant turned out to be much more transmissible than other variants, the country could face “hard choices”.

It comes as minutes released by the Scientific Advisory Group for Emergencies (Sage) from their meeting on Thursday said “it is highly likely that this (Indian) variant is more transmissible than (Kent) and it is a realistic possibility that it is as much as 50% more transmissible”.

The experts warned there will be an even faster increase in cases if restrictions are lifted, suggesting a peak of infection can be expected after Monday’s easing, and certainly at step four of the road map in June, when all legal limits on social contact are due to end.

Sage said: “If this variant were to have a 40-50% transmission advantage nationally compared to (Kent), sensitivity analyses in the modelling of the road map in England indicate that it is likely that progressing with step three alone (with no other local, regional, or national changes to measures) would lead to a substantial resurgence of hospitalisations (similar to, or larger than, previous peaks).

“Progressing with both steps three and four at the earliest dates could lead to a much larger peak.”

During the Downing Street briefing, the Prime Minister said: “I do not believe that we need, on the present evidence, to delay our road map and we will proceed with our plan to move to step three in England from Monday.

“But I have to level with you that this new variant could pose a serious disruption to our progress and could make it more difficult to move to step four in June.”

Later, he added: “This doesn’t mean that it’s impossible that we will be able to go ahead with step four.

“I don’t think that’s the case at all. But it does now mean there’s the risk of disruption and delay, and delay to that ambition, and we have to be utterly realistic about that.”

However, the Prime Minister said there was no evidence that a rise in cases of the Indian variant was translating into unmanageable pressures on the NHS in terms of hospital admissions, which at the moment remain “broadly flat”.

Data from Public Health England (PHE) shows a rise in cases of the Indian variant of concern from 520 to 1,313 this week in the UK, with the agency saying infections were “rising in the community”.

England’s chief medical officer, Professor Chris Whitty, told the briefing the variant was “quite widely seeded in a number of parts of England and indeed elsewhere in parts of the four nations of the United Kingdom”, and could overtake the Kent strain to become dominant in the UK.

He warned that if the variant proves to be a lot more transmissible than other variants, the UK could see “a really significant surge” in Covid-19 cases, adding: “That’s a really critical question to which we do not yet have the answer.”

Prof Whitty said the UK might be seeing a delay in the impact of the new variant or that vaccines were holding it at bay.

He said: “It could be that it is initially circulating in younger ages, because that’s what has always happened previously.

“Younger people mix more and the initial circulation is in younger ages, and then it moves up the age range – so maybe it’s just a delay because of that.

“Or maybe it’s a delay because the vaccine is actually providing a firebreak, a barrier to reduce the transmission up the ages, into those who are most vulnerable, clearly the second of those is by far the more preferable.”

In a bid to dampen the effects of the surge in cases, Mr Johnson announced that people over 50 and the clinically vulnerable will have their second doses of a Covid vaccine accelerated.

Mr Johnson said second doses – which give people maximum protection against Covid-19 – will be brought forward from the planned 12-week interval to eight weeks.

He said: “I believe we should trust in our vaccines to protect the public whilst monitoring the situation as it develops very closely because the race between our vaccination programme and the virus may be about to become a great deal tighter and it’s more important than ever, therefore, that people get the protection of a second dose.

“So following advice from the Joint Committee on Vaccination and Immunisation we will accelerate remaining second doses to the over-50s and those clinically vulnerable right across the country so those doses come just eight weeks after the first dose.”

Prof Whitty said the continued rollout of first doses to younger age groups is not expected to be delayed by accelerating second doses to the over-50s and vulnerable.

The PM said the army would be deployed in parts of the North West which have suffered some of the highest rates of the Indian variant, and will hand out tests to help the surge testing efforts.

Surge testing is being carried out in several places in England including areas of Bolton, Blackburn, Sefton and London.

Mr Johnson also urged people to “think really carefully” about the risk to loved ones, “especially if they haven’t had that second dose or if it hasn’t yet had time to take full effect.”

He added: “I want us to trust people to be responsible, and to do the right thing. That’s the way to live with this virus while protecting the NHS and restoring our freedoms.

“It’s very clear now we’re going to have to live with this new variant of the virus for some time so let’s work together, and let’s exercise caution and common sense.”

The Prime Minister said that the UK’s surveillance data is now so advanced, that it would see well ahead of time if the NHS was likely to come under unsustainable pressure.

“That gives us the confidence to continue moving forwards for now,” he said.

However, he urged people to “think twice” ahead of travelling to areas with higher incidences of the Indian variant and staying with family and friends within those areas.

“We want people in those areas to recognise that there is extra risk, an extra threat of disruption to progress caused by this new variant and just to exercise their discretion and judgment in a way I’m sure that they have been throughout this pandemic,” he said.

Shadow health secretary Jonathan Ashworth accused Mr Johnson of a “reckless failure to protect our borders”.

He added: “Only a few weeks ago we urged Matt Hancock to designate this a variant of concern and respond with speed and resolve.”

UKHospitality’s chief executive Kate Nicholls said it was “vital” financial support was offered if step four of the road map was disrupted by any surge of the Indian variant of coronavirus.

She said: “The Prime Minister’s statement has introduced an element of doubt surrounding the dropping of restrictions on June 21.

“The situation will clearly be closely monitored but should the road map timings slip, it is vital financial supports are forthcoming, and that business rates are postponed until October.”