Conspiracy theories that spread online, such as QAnon and Pizzagate, can inspire domestic terrorists to "commit criminal and sometimes violent activity", according to the FBI.

The warning comes from an agency bulletin at the FBI’s Phoenix field office, published by Yahoo News.

The FBI document specifically names a rightwing conspiracy theory known as QAnon, which was spread on anonymous internet forums 4Chan and 8Chan.

Its premise is that an individual or a group of people known only by the moniker ‘Q’ are working from inside the government against the 'deep state'.

Q claims to have top level security clearance and has allegedly teamed up with Donald Trump to bring down a global cabal of evil government figures and celebrities.

Trump supporters have been at rallies wearing shirts and hats that feature the letter “Q" in support of the theory.

It also names Pizzagate, the conspiracy theory that high-ranking democratic officials were involved in a child trafficking ring centred at the Comet Ping Pong pizza restaurant in Washington DC.

The bulletin is the “first FBI product examining the threat of conspiracy theory–driven domestic extremists” and lists examples of attempts or acts of violence due to fringe theories.

They include a gunman who killed 11 people at the Tree of Life Synagogue after shouting "all Jews must die" during a circumcision ceremony in 2018.