The change, which means an attack is now seen as highly likely, comes the day after a gunman in Vienna identified as a convicted jihadist killed four people in a rampage overnight. France has also suffered three attacks in recent weeks.
"This is a precautionary measure following the terrible instances that we've seen in France last week, and the events that we saw in Austria last night," Patel said in a televised statement
She said the public should not be alarmed and that the change in threat level was not based on any specific threat.
The new threat level means an attack is highly likely, according to the government's classification system. The previous "substantial" level meant an attack was likely.Britain's threat level is assessed by the Joint Terrorism Analysis Centre which is accountable to the domestic intelligence agency MI5 and made up of representatives from 16 government departments and agencies.