20 February 2011 Last updated at 21:21 ET

7/7 inquests: MI5 officer to give evidence

A senior member of MI5 will give evidence later at the inquests into the deaths of 52 people killed in the 7/7 terrorist bombings in London in 2005.

The officer, who will be referred to as Witness G, will be asked whether the attacks could have been prevented.

The bereaved families in court will be able to see him but reporters in a nearby annexe will only hear his voice.

Four suicide bombers detonated their devices on three Tube trains and a double decker bus on 7 July 2005.

Witness G will be asked about a key moment months before the bombings when the security service came across two of the terrorists during an investigation into another plot.

'Significant day'

Many of the relatives of those who died want to know why those under surveillance were not subjected to detailed scrutiny.

MI5 has always maintained it did not uncover any intelligence that would have identified the pair as potential suicide bombers.

BBC correspondent Peter Hunt says it will be a significant day as the senior MI5 officer will sit in the witness box and be questioned in public.

Last month, the coroner Lady Justice Hallett granted a request from Home Secretary Theresa May for Witness G to give evidence with anonymity.

But she refused to rule that the witness should be screened from the families of those who died.

The attacks were carried out by suicide bombers Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Jermaine Lindsay, 19.

They targeted Tube trains at Aldgate, Edgware Road and Russell Square and a bus in Tavistock Square.