Boris Johnson cautioned on Wednesday against an early lifting of England’s lockdown restrictions, as the UK reached the milestone of vaccinating more than 10m people against coronavirus.

The government reported that a further 374,756 people had been given their first dose of the vaccine in the latest 24-hour period, taking the total to 10,021,471.

With the UK vaccine programme showing early momentum, the government looks on track to hit its target of 15m vulnerable Britons having jabs by February 15.

But speaking at a Downing Street press conference, Mr Johnson declined to commit the government to reopening schools in England before March 8, which he has previously said is the earliest possible date.

“We think this is the prudent and cautious approach,” he said. “It is much better to stick to that.”

Mr Johnson also said the level of Covid-19 infection in the population remained “alarmingly high”.

Chris Whitty, England’s chief medical officer, said the UK was now on a downward trajectory in terms of the numbers of Covid-19 cases, hospital admissions and deaths, adding that his colleagues thought “we are past the peak”.

But he also suggested that the premature easing of the lockdown would mean the NHS could “get into trouble incredibly fast”.

The government’s target of vaccinating 15m Britons by mid-February is focused on four priority groups: people in care homes, NHS workers, the clinically vulnerable and those aged over 70.

Mr Johnson said he would announce details on vaccinating the next five groups, including all those over 50, around February 15, with a road map for easing the lockdown the following week.

Prof Whitty suggested that only after the nine groups had been vaccinated could a substantial easing begin.

The prime minister said the fall in Covid-19 cases was “pretty uniform” across England, and therefore he expected the country to leave lockdown as a whole, rather than regions being put back into a series of tiered restrictions that were first introduced last year.

“It feels at the moment that we will be going down in tiers nationally,” he added, while stressing no decisions had been taken.

Taking a nationwide approach will aid Mr Johnson’s desire to move gradually out of the lockdown restrictions.

Allies said he was eager that this be the final lockdown in England, and would resist pressure from Conservative MPs for a more rapid lifting of restrictions that might overwhelm the NHS.

One reason for taking a nationwide approach is the way a UK variant of coronavirus identified in December has spread rapidly across the four constituent nations.

“The new variant is spreading so quickly that we have to think in a national sense,” said one government official.

Mr Johnson is also aware that returning England to a tiered system of restrictions risks pitting parts of the country against each other, particularly after areas of northern England were subjected to extended periods of de facto lockdown last year.

“Boris wants to move as one nation together out of this last lockdown,” said one ally of the prime minister.

The government is also confident that England’s test and trace programme, which has previously been criticised for its shortcomings, can in the future be deployed at a hyperlocal level to identify and contain outbreaks.

This is being done this week in response to a South African variant of coronavirus that has been found in at least eight locations in England.

But Dido Harding, head of the test and trace programme, told MPs the system was “not a single silver bullet”, adding: “We’ve seen the virus mutate, the new variant emerge, which was something that none of us were able to predict.”

Some Whitehall officials said data suggesting the Oxford/AstraZeneca vaccine now being used in the UK could reduce transmission of coronavirus by 67 per cent was a “game changer”, and could see lockdown eased “much quicker than we had previously hoped”.

Martin McKee, professor of European public health at the London School of Hygiene and Tropical Medicine, said the findings “will definitely help” but cautioned that it was insufficient to reach the herd immunity threshold, especially given the increased transmissibility of the UK variant.

Prof McKee added it would “probably bring forward when we can think about opening up but it’s really difficult to be sure just when”.

Meanwhile the government is backing the first study in the world to look at whether using different vaccines for the first and second doses improves people’s immune responses. This could, for example, involve using Oxford/AstraZeneca’s vaccine for the first dose, followed by BioNTech/Pfizer’s alternative for the second.

The government is providing £7m towards the study, run by the National Immunisation Schedule Evaluation Consortium.

Officials said there were no current plans to change the regimen under which people receive the same vaccine for both doses. Results from the study are expected to be released in the summer.