Video report by ITV News Political Correspondent Romilly Weeks
The government's Chief Medical Officer Chris Whitty has warned the UK has a "long winter ahead" as Covid infection rates in local lockdown areas head in the "wrong direction".
He said there has been a "significant rise" in test positivity, despite the new rule of six, with the increase "accelerating quite rapidly" in some areas such as North East and North West England.
Infections are rising "rapidly" among the young, he said, but the spread is reaching older, more vulnerable members of the population.
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Watch the full Covid briefing here:
Prime Minister Boris Johnson, speaking alongside Mr Whitty at the Downing Street news conference, said the country was at a "critical moment" and that he would not hesitate to a bring in new measures if required.
Mr Johnson said tougher restrictions would be imposed if the current ones did not have an impact on the spread of coronavirus, adding: "If we work together now, we give ourselves the best possible chance of avoiding that outcome."
He said the UK is experiencing a resurgence , with around 15% of tests coming back with positive results.
Chief Scientific Adviser Sir Patrick Vallance said: "It is very clear that rates are still going up. "
He added: "And the increases that Chris (Whitty) has described in some areas are of concern will lead to further problems."
ITV News Science Editor on the latest rise in coronavirus cases
Sir Patrick admitted the UK does not currently have the pandemic "under control at the moment".
Repeating a message the government's top scientists have issued throughout the pandemic, Sir Patrick said: "The decisions that need to be made are clearly very difficult, none of them are risk free, they all come with some cost and that's human cost in some form or another.
"Science can provide advice on the situation, ministers need to make decisions as to when and how to act."
Reacting to the government's briefing on Wednesday, Labour leader Sir Keir Starmer criticised Number 10's lack of a "clear strategy for keeping that infection rate down".
On whether the UK could be facing a second national lockdown, the Labour leader called for an "urgent review" into whether "local restrictions are working in the way intended" - citing that just one of the areas with local restrictions has successfully come out of them.
Describing the rise in cases as "very concerning", Sir Keir called on the government to "urgently fix" its test, trace and isolate system.
Speaking at the briefing, the PM said the UK is on course to reach capacity for 500,000 tests a day by the end of October.
He said he is "absolutely confident" that improved testing, better treatments and the "prospect of a vaccine" will allow the UK "get through this".
As the press conference was taking place, MPs granted the government permission to carry on imposing Covid-19 restrictions as the Coronavirus Act passed the Commons with a majority of 330 votes to 24.
It allows the government to continue with new restrictions, but requires MPs to have a vote on matters of national significance.He called on the British public for "collective forbearance, common sense and willingness to make sacrifices" in the battle against Covid-19.
The update came as the UK recorded 7,108 daily cases of coronavirus - the second highest number of positive cases since the pandemic began.
It was the 100th Downing Street press conference on the coronavirus pandemic and many more are likely to follow after the PM announced they would be held more regularly.