Boris Johnson has insisted the UK is at the "front of the pack" to receive a breakthrough vaccine, but said the UK could not "rely on this news as a solution".
The prime minister confirmed the UK has purchased enough doses of the Pfizer vaccine for a third of the population, but warned the breakthrough development still had "several hurdles" to clear.
Speaking at a Downing Street press briefing, Johnson said the "promising" interim results showed the vaccine had a 90% success rate in protecting people from the virus, but that scientists were waiting to see further safety data.
It comes after NHS England confirmed a further 167 people had died in hospital from the virus in the last 24 hours, with a further 20,572 new cases recorded.
"What I can say is that if and when this vaccine is approved, we, in this country, will be ready to start using it," Johnson said.
"Earlier this year the UK government ordered 40m doses of the Pfizer vaccine – enough for about a third of the population, since you need two doses each.
"That puts us towards the front of the international pack on a per capita basis – and I should add we’ve ordered over 300m doses from 5 other vaccine candidates as well.
"If the Pfizer vaccine passes all the rigorous safety checks and is proved to be effective then we will begin a UK-wide NHS led programme of vaccine distribution."
But the PM insisted the news was not a "slamdunk" as he warned ministers would not "slacken our resolve" in bringing down the infection rate.
"I must stress these are very early days. We have talked for a long time about the distant bugle of the scientific cavalry coming over the brow of the hill. I can tell you that tonight, that toot of that bugle is louder, but it’s still some way off," he added.
"We absolutely cannot rely on this news as a solution and the biggest mistake we can make now would be to slacken our resolve at a critical moment.
"The biggest mistake we can make is to slacken our resolve.
"Now it’s more important than ever to follow the rules, I know it’s been a tough first weekend of these autumn restrictions.”
Meanwhile, England's deputy chief medical officer Jonathan Van-Tam said the vaccine, which is likely to be months away from approval, would offer no help in tackling a second wave of the virus during the winter.
"This is like getting to the end of a play-off final... the first player goes up and scores a goal. You haven't won the cup yet but it tells you the goalkeeper can be beaten," Mr Van-Tam said.
"We don't know what this means yet for when we can get life back to normal.
"It would be a colossal mistake for anyone of us to relax at this stage."
John Johnston is a political reporter for CSW's sister title PoliticsHome, where a version of this story first appeared.