The Daily Mail reported that security officials raised the specific threat to the transport network, which means that an attack is 'highly likely'.
"The threat level to the UK is at severe, which means that an attack is highly likely, and has been since January 2010. We will police accordingly and use a range of covert and overt tactics which remain under constant review," a spokesman for the Metropolitan Police said.
"We continue to encourage the public to remain vigilant and aware and to trust their instincts and report any suspicious behaviour which may be terrorist-related," he added.
Transport police were reportedly ordered to cancel their leave so that extra officers could be deployed across the Tube and busy airports, including Luton and Heathrow. Security is likely to be increased, with potential extra restrictions for travellers.
Following the 7/7 atrocity that killed 52 people, no successful attack on the London transport has been made so far.
It is also reported that the emergency services have been briefed about how to respond to a copy of the 2008 Mumbai machine gun attacks.
The increased terror alert, confirmed in a memo sent to staff at airports and Transport for London yesterday, follows a string of warnings that Britain could face a new Al Qaeda-inspired attack. Potential targets included the London Eye tourist attraction, the Stock Exchange and Mayor Boris Johnson, it was claimed.
In 2008, terrorists rocked the Indian business capital, Mumbai, in which 175 people were killed and hundreds others were injured.