Boris Johnson said he is “very confident” in the coronavirus vaccines after concerns were raised that the Oxford/AstraZeneca jab may be less effective against the South African variant.

But the Prime Minister did not rule out the strain could delay the relaxation of lockdown restriction, instead insisting he has “no doubt that vaccines generally are going to offer a way out”.

Experts warned on Monday it is “very possible” the strain is already quite widespread in the UK after a study of around 2,000 people suggested the Oxford jab only offers minimal protection against mild disease of the South Africa variant.

The study, due to the young age of participants, could not conclude whether the vaccine worked against severe disease.

Some 147 cases of the South African variant have so far been identified in the UK, with experts warning these are likely to be the “tip of the iceberg” due to the fact they are the result of random checks on 5% to 10% of all positive tests.

During a visit to a coronavirus test manufacturing facility in Derby, the Prime Minister told reporters: “We’re very confident in all the vaccines that we’re using.

“And I think it’s important for people to bear in mind that all of them, we think, are effective in delivering a high degree of protection against serious illness and death, which is the most important thing.”

Earlier Dr Mike Tildesley, a scientist advising the Government, warned that restrictions may be needed for longer if the South African variant does turn out to be prevalent in the UK.

Mr Johnson did not rule out that there could be a delay to easing restrictions if the Oxford/AstraZeneca has a reduced effect on preventing infections, and therefore on hindering the transmission of Covid-19.

But he said: “They remain of massive benefit to our country and to the population as we go through the pandemic and I’ve no doubt that vaccines generally are going to offer a way out.

“With every day that goes by, you can see that medicine is slowly getting the upper hand over the disease.”

Mr Johnson insisted the nation will be “relying very much on border controls” to protect against new strains, but that they will be “most effective” when infection rates are brought down.

His comments came as Downing Street admitted that no formal contracts have been awarded to hotels for arrivals from “red list” countries to quarantine in – with the policy due to be introduced in England on February 15.

Meanwhile, health minister Edward Argar said that booster jabs to tackle new strains are already in the pipeline.

Dr Tildesley, a member of the Scientific Pandemic Influenza Group on Modelling (Spi-M), said “surge testing” currently taking place must be effective to stop the South African strain proliferating in the UK, but when asked if it may already be quite widespread, he replied: “It’s very possible.”

The Warwick University researcher added to BBC Radio 4’s Today programme that lockdown restrictions may be needed for longer if the variant does turn out to be prevalent in the UK and can be transmitted by vaccinated individuals.

He said those developments would have “significant implications” because there would be “a lot of people that could potentially get infected and could potentially pass it on and it may mean that more restrictions might be needed for longer if we can’t get on top of this”.

However, Mr Argar said Professor Sarah Gilbert, from Oxford, who designed the Covid vaccine, had said “there wasn’t anything she could point to that caused her to be concerned that it would not be effective against severe forms of the illness, hospitalisation and death from the virus”.

He told BBC Breakfast that dealing with severe disease and people needing to go to hospital was “the key thing we are seeking to tackle here at this point”.

Mr Argar said booster jabs were already being developed to tackle variants, telling Sky News: “What we would all expect is every year we have our flu jabs, it would not be unreasonable to suggest something similar here.”

The minister said the virus “will always try to outwit us”, adding: “We’ve just got to make sure we get ahead of the game and we outwit it.”

Stephen Evans, a professor at the London School of Hygiene and Tropical Medicine, said he was “reasonably confident” that current vaccines would protect against serious disease from the variant.

But he told BBC Radio 4’s The World At One programme that the number of cases of the strain was “probably here in dramatically more numbers” than had been identified so far.

The variant accounts for around 90% of new coronavirus cases in South Africa, which has put its rollout of the Oxford/AstraZeneca vaccine on hold.

Professor Salim Abdool Karim, the head of South Africa’s Ministerial Advisory Committee on Covid-19, said the Oxford study could not show that the vaccine was effective against all levels of seriousness of the South African variant.

He told Today: “What the study results really tell us is that, in a relatively young age group demographic – with very low prevalence of morbidities such as hypertension and diabetes etc – the vaccine does not protect against mild to moderate infection.”

The professor told Times Radio he does not know if the Oxford/AstraZeneca vaccine will be effective in “preventing severe disease”.

“Our problem is we don’t know if the AstraZeneca vaccine will be effective in preventing severe disease and hospitalisation in the population. That’s why we felt we should hold until that information becomes available,” he said.

“We’re taking a two-step approach, the first is to vaccinate probably around 100,000 individuals and assess what the hospitalisation rates are.

“Once we’re confident the hospitalisation rates are low with the AstraZeneca vaccine, then we proceed to roll out the remaining million doses we have.”