But the "severe general" alert was not connected to US President George Bush's forthcoming UK visit, the BBC said on Saturday.
Security measures for Bush's visit were already "without precedent", according to John Stevens, the head of London's Metropolitan police.
The additional alert was for the security services, including police, intelligence and the army and was not directed at the public.
The BBC said the threat was of a general nature concerning alleged plans by al-Qaida supporters from North Africa.
The Home Office, Britain's ministry of the interior, declined to comment. A spokeswoman said it never discussed threat levels unless there is a specific threat.
She added, "If there is a specific threat, the government would not hesitate to issue an alert."
The alert comes just one week after al-Qaida carried out a bomb attack on a residential compound in the Saudi capital Riyadh that left 17 people, mostly expatriate Arabs, dead.
The group has previously named the UK as a possible target because of the government's support for US intervention in the Middle East and the wider Muslim world.