from the BBC

A plot to blow up planes in flight from the UK to the US and commit “mass murder on an unimaginable scale” has been disrupted, Scotland Yard has said.
…
The threat level to the UK has been raised by MI5 to critical after the arrests in London, High Wycombe and Birmingham.

Critical threat level – the highest – means “an attack is expected imminently and indicates an extremely high level of threat to the UK”.