The officials have put this threat at the top end of 'severe', the third of four risk levels which rate an attack 'highly likely', sources said.
"The Government is right that the horrific events in Mumbai have highlighted the need for a different strategy in counter-terrorism. No part of the UK is free from threat - and we know that terrorists want soft targets," Deputy Home Secretary Chris Grayling said.
The British Government is planning to launch a revised counter-terrorism policy next week, which would includes guidelines for security officials for safe guarding important buildings in the country, The Daily Telegraph reports.
The threats come days before London is scheduled to host the G-20 summit.
Anti-terrorist officials also are concerned that cities other than London could be targeted.
"The Home Office was acutely aware that the focus on securing London ahead of the Games could displace attacks onto other parts of the UK," a senior official said.
Sources added that the Gordon Brown administration is chalking out plans to identify and find ways of deter young Muslims from being radicalised and persuaded to take on terror activities.
A tight vigil is being maintained by security officials to check the smuggling of arms and ammunition through ports and remote airfields, where security is considered to be more lax.