Britain's terrorism threat level has been raised to "severe", according to Home Secretary Priti Patel, meaning an attack is now seen as highly likely.

The change comes after a gunman in Vienna killed four people in a terror attack overnight.

"This is a precautionary measure and is not based on any specific threat," Ms Patel said on Twitter.

"The British public should be in no doubt that we will take the strongest possible action to protect our national security."

She did not mention the Vienna attack in her statement. 

The new threat level means an attack is highly likely, according to the UK government's classification system. The previous "substantial" level meant an attack was likely. 

The UK decision was made by the Joint Terrorism Analysis Centre (JTAC), which conducts a formal review of the threat level every six months, independent of government ministers.

JTAC's membership includes the security services MI5 and its overseas counterpart MI6, and police, and assesses all intelligence relating to terrorism at home and abroad.

The level was briefly raised on two occasions to "critical", the maximum level indicating an attack is "highly likely in the near future", in May and September 2017.

The move was in response to conflicts in Syria and Iraq. Manchester and London also both suffered deadly attacks that year.

It was also changed to "critical" in August 2006 after the security services said they had foiled a plot to blow up flights between Britain and the United States.