The State Department has elevated the UK to "do not travel" status amid a surge in Covid-19 cases in Britain.
Following the updated advice from the Centres for Disease Control and Prevention (CDC), Britain has been placed at the highest warning level, meaning Americans should not visit except in an emergency.
It comes after Joe Biden last week suggested that travel between the US and the UK was close to reopening, promising an update on talks "within the next several days".
The UK recorded more than 48,000 new cases on Monday, with cases up 52 per cent week on week, and there is concern in Washington that the virus could spread even among the vaccinated.
The State Department has followed the CDC advice which warns Americans that "because of the current situation in the United Kingdom, even fully vaccinated travellers may be at risk for getting and spreading Covid-19 variants".
The UK is now at Level 4, or "Very High", in the CDC’s Covid warning levels.
The State Department’s advisory said: "Do not travel to the United Kingdom due to Covid-19. Exercise increased caution due to terrorism.
"The CDC has issued a Level 4 Travel Health Notice for the United Kingdom due to Covid-19, indicating a very high level of Covid-19 in the country.
"There are restrictions in place affecting US citizen entry into the United Kingdom."
The move is likely to be a setback to Britons hoping to visit the US, which has been shut to the majority of British travellers since March 2020.