A massive police operation is under way in London - two weeks since the failed 21 July bombings and four since the 7 July attacks in which 56 people died.

More than 6,000 officers, many armed, are patrolling stations and key sites, as the Piccadilly Line has reopened.

A high-visibility presence is aimed at reassuring people while undercover officers mingle with commuters on Tubes and buses trying to spot bombers.

The first person charged in connection with the attacks is due in court.

Metropolitan Police Assistant Commissioner Tarique Ghaffur said work on major murder investigations had "slowed to a trickle", because so many specialist detectives had been moved to tackle terrorism.

Extra officers have been brought in from outside the capital and retired officers persuaded to return to help with the anti-terror work, he told BBC News.

"Any help we are able to call upon we will call upon.

"But the longer-term implication, even of bringing in retired officers, has to be considered because it has budgetary implications."

London Underground said all stations on the Tube network were now open.

The Piccadilly Line had been suspended from Hyde Park Corner to Arnos Grove and from Rayners Lane to Uxbridge.

The Hammersmith and City Line, also shut since 7 July, reopened on Tuesday leaving the Circle Line the only one to remain closed.

Ismael Abdurahman, 23, of Newport Street, Kennington, south-east London, will appear before Bow Street Magistrates' Court charged under the Terrorism Act in relation to the failed bomb attacks in London on 21 July.

Arrested on 28 July, he is accused of withholding information that may have helped catch a person involved in terrorism.

The allegation is understood to relate to the Shepherd's Bush bombing, with suspect Osman Hussain currently being held in Italy.

There are a further 14 people being held in connection with the attacks.

PHOTO CAPTION

Police officers patrol Russell Square Underground station after it reopened for the first time following the bomb attacks of July 7 in central London, August 4, 2005. (REUTERS)