The breakthrough of a coronavirus vaccine has "cleared one significant hurdle but there are several more to go", Boris Johnson has said.

The PM said early findings showing a jab could prevent 90% of people getting Covid-19 were positive, but added it was "very, very early days".

He warned people not to "rely on this news as a solution" to the pandemic.

"The biggest mistake we could make now would be to slacken our resolve at a critical moment," he said.

England's deputy chief medical officer, Prof Jonathan Van-Tam, said he was "hopeful" the first vaccine could be seen by Christmas.

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Speaking alongside Mr Johnson at a Downing Street news conference, Prof Van-Tam said that there was more work to be done before it became available to the public.

"This is a very important scientific breakthrough. I am certain of that," he said.

"I am hopeful because of all that, but not yet certain that we could begin to see some vaccine by Christmas."

And he said age would be the "biggest priority" when drawing up a list of who would be able to access the new vaccine.

Prof Van-Tam added that the development was similar to "getting to the end of the playoff final, it's gone to penalties, the first player goes up and scores a goal".

"You haven't won the cup yet, but what it does is it tells you is that the goalkeeper can be beaten," he said.