Boris Johnson on Monday warned the government was taking a “cautious” approach to lockdown easing, as he hailed the vaccination of 15m people against coronavirus in the UK as an “unprecedented national achievement”.

The prime minister urged the public to be both patient and optimistic, saying now was not the time to relax because the overall threat of the virus to the public remained high.

Speaking at a Downing Street press conference, Johnson said he was determined that the current lockdown in England should be the last, adding that he wanted the easing of restrictions to be “cautious but also irreversible”.

However, he also conceded he could not guarantee that the country would not be put into another lockdown in the future.

“I’m very hopeful that we’ll be able to go ahead and open things up, but to say I can give an absolute cast-iron guarantee that we won’t face further difficulties and have to think harder and deeper about some problems, no at this stage I can’t,” said Johnson.

Next Monday the prime minister is due to set out a government plan for lifting the lockdown restrictions in England, starting with the expected reopening of schools on March 8.

He is coming under growing pressure from lockdown-sceptic Conservative MPs to outline a swift timetable for ending restrictions.

At the weekend, more than 60 Tory MPs who belong to the Covid Recovery Group urged Johnson to lift all restrictions if the government hits its target for nine priority groups — including all those aged over 50 — to have been offered a first dose of the vaccine by the end of April.

But the prime minister suggested he would not take any risks, saying he was still waiting for more data on the way in which vaccines prevented infection. “The threat from this virus remains very real,” he said.

The government confirmed it had met the initial target of its vaccine programme by offering a first dose to almost 15m Britons in the top four priority groups, including all those aged over 70, by February 15. About nine in 10 people aged over 70 took up the offer of a jab.

The government reported on Monday that 15.3m Britons have had a first dose of the vaccine.

The NHS has now begun the second phase of the vaccine programme, focused on the next five priority groups. These include Britons aged over 65, those with underlying health conditions aged between 16 and 64, and adult carers.

The government aims to have offered vaccinations to all 32m people within the nine priority groups by the end of April.

However, there are concerns surrounding the limited take up of vaccines among ethnic minority communities as well as care home workers.

A study by University Hospitals of Leicester NHS Trust found that 59 per cent of South Asian staff and 37 per cent of black workers had received a first dose of the vaccine, compared to 71 per cent of white employees.

Health secretary Matt Hancock said one-third of social care staff in England were yet to be vaccinated despite being within the first four priority groups.

Simon Stevens, chief executive of NHS England, said there was a “huge effort” involving the NHS, faith leaders and local communities to overcome vaccine hesitancy, which he blamed partly on disinformation.

“Part of what we are up against is a duel epidemic — we are up against the pandemic of Covid and we are up against a pandemic of disinformation and the deliberate sowing of mistrust and we have to fight both with equal vigour,” he added.