London: A fresh terror alert has been issued in London amid fears of a Mumbai-style attack on airports and transport system, a media report said Friday.
Thursday's alert meant that an attack was 'highly likely', the Daily Mail reported.
Transport police were ordered to cancel their leave so extra officers could be deployed across the Tube and busy airports, including Luton and Heathrow, it said.
Security is likely to be ramped up, with potential extra restrictions for travellers.
There has not been a successful attack on the London transport system since 52 were killed July 7, 2005.
It also emerged that the emergency services have been briefed about how to respond to a copy of the 2008 Mumbai machinegun attacks.
Terrorists spread out across the Indian city - slaughtering more than 160 people at a string of busy locations, including a busy train terminus.
The increased terror alert, confirmed in a memo sent to staff at airports and transport for London, follows a string of warnings that Britain could face a new Al Qaeda-inspired attack.
Last month, the police and security services disrupted an alleged plot for a Christmas bomb blitz in London. Nine suspects appeared in court over Christmas in connection with the allegations.
In October, an attempt was made to bring down at least two cargo jets with devices packed with the powerful explosive hidden inside printer cartridges.
One of the cartridges, which was timed to explode in a Lockerbie-style atrocity, was discovered after the plane landed at East Midlands airport.
Home Secretary Theresa May said the group behind the plot, the Yemen-based Al Qaeda in the Arabian Peninsula, already had operatives on the ground in Britain.
Details of the latest alert emerged following the increase in the specific threat faced by the transport and aviation 'sub sector'.
MI5 and the Home Office publish the nationwide level terror threat, which is currently rated 'severe'.
But JTAC, the joint terrorism analysis centre based inside MI5, also compiles threat levels relating to individual elements of Britain's infrastructure.
Normally, these threat levels are classified. However, the latest threat level has been raised to state that an attack is now considered highly likely.
It points to intelligence being received about a specific threat. Whitehall sources said the rail travel alert related to London rather than stations nationwide.
Two former SAS commanders have warned that if terrorists did manage a Mumbai-type strike in Britain, the country would face a 'slow motion' Sep 11.