Rumours suggesting London's Tube network will be targeted in a terrorist attack on Monday are "completely untrue", police say.
Messages claiming every single Metropolitan Police officer had been called into work from 4am on Monday in response to a specific threat to the London Underground spread across social media last night.
But a police spokesman told ITV News the message was "completely untrue", and would not be discussed further.
The London boss of the British Transport Police also rubbished the claims on Twitter, telling the public to "keep calm and carry on".
The rumours come after the UK's terror threat level was raised from substantial to severe on Friday - meaning a terrorist attack was "highly likely" but "not imminent".
Home Secretary Theresa May said the increase was related to the conflicts in Syria and Iraq, and no specific threat had been made.
David Cameron warned that the rise of the Islamic State meant the UK faced a "greater and deeper threat than we have known before".
He said extra security measures such as increased patrols were now being enforced, but said people should "continue to go about their daily lives in a normal way".