The plan to lift all Covid restrictions later this month could be blown “off track” by surging infections, the government has admitted, with the case for pressing ahead now “ambiguous”.

As the number of new cases hit a two-month high, Boris Johnson expressed his concerns that the elderly and vulnerable could be put at risk if what remains of the lockdown is lifted too soon, despite the success of the vaccination programme.

In his first comments for almost a week, the prime minister again insisted he could “see nothing in the data at the moment that means we can’t go ahead” with the remainder of the roadmap.

But he then added: “What we need to work out is to what extent the vaccination programme has protected enough of us, particularly the elderly and vulnerable, against a new surge – and there I’m afraid the data is still ambiguous.”

Matt Hancock, the health secretary, broke with the usual cautious optimism from ministers, saying instead: “There is nothing in the data to suggest we are definitively off-track,” implying he feared that such evidence may yet emerge.

His downbeat tone came as 4,330 new confirmed coronavirus cases were recorded for the latest 24-hour period – up from 3,165 on Tuesday, and the highest tally since 4,479 were logged on 1 April.

Infections are now rising by 35 per cent week-on-week, and hospital admissions by 17 per cent. The 12 deaths – while a low number – marked a change from the euphoria of a day earlier, when no deaths were recorded.

The increases reflect the spread of the Indian variant in hotspots, particularly in northwest England, with the data being “interrogated” in lengthy meetings, the prime minister said.

In better news, the proportion of deaths involving coronavirus in England and Wales is at an eight-month low – and three-quarters of UK adults have received the first dose of a vaccine.

A decision on whether to press ahead with step 4 is due to be taken on 14 June, although Mr Hancock suggested it could come slightly earlier.

He also did not rule out a partial lifting of the remaining restrictions – perhaps retaining mask-wearing and work-from-home guidance, while ending social distancing – in a speech hailing the success of the vaccination programme.

And he acknowledged that “the vast majority of people” in hospital are not “fully vaccinated”, when questioned about research revealing limited protection from the new variant after a single dose of vaccine.

Although ministers are speeding up second doses for over-50s, attempting to reach all those in that age group by 21 June, that would still leave younger people without optimum protection until much later in the summer.

“We can see the number of cases has been rising in the last couple of weeks, but we can also see that the vast majority of people who have ended up in hospital are not yet fully vaccinated,” Mr Hancock said.

“And the critical question is how much that link is broken? And that’s how we’ll make the decision, ahead of 21 June.”

The health secretary added: “It is too early to make the decision about 21 June (step 4 on the roadmap). We’ll make that decision based on more data in the next week to 10 days.”

The questions came as a growing chorus of scientists urged the government to slam the brakes on existing plans to lift restrictions, because of the deteriorating picture.

Many point out that one of the four tests previously set down as a requirement for lifting remaining restrictions is that no new variant is taking root.

Mr Johnson was also strikingly cautious about whether more countries will be added to the “green list” for quarantine-free travel on Thursday.

“We’ve got to be cautious and we’ve got to continue to put countries on the red list, on the amber list, when that is necessary,” he said.

One analysis found that, of 315 local areas in England, 68 per cent saw a rise in Covid rates over the week to 29 May, while only 29 per cent experienced a fall.

Blackburn with Darwen, in Lancashire, continues to have the highest rate, with 653 new cases – the equivalent of 436.2 cases per 100,000 people, and up from 314 the previous week.

It had the second biggest week-on-week rise, behind Rossendale (up from 138.5 to 312) and ahead of Ribble Valley (29.6 to 151.1), Hyndburn (80.2 to 196.2) and Salford (41.3 to 123.2).