Parliament agreed in March to double the previous 14-day period that police can hold terrorist suspects. The Lower House previously rejected a proposal from Prime Minister Tony Blair's government for a 90-day period.
But the change could not be implemented before the adoption of a new code of practice dealing with the detention, treatment and questioning of terrorist suspects pending charge and trial.
On Friday, Britain marks the first anniversary of its worst-ever terrorist incident.
Fifty-two people were killed hundreds were injured when four suspects exploded bombs on three London underground trains and a double-decker bus.
Terror dossier in ditch
Meanwhile a sensitive dossier featuring details of the British government's response to a major terrorist attack has been found in a roadside ditch.
The 46 page Ministry of Defence file was discovered in a gym bag and handed to The Sun newspaper by a member of the public.
The bag, also containing a ministry security pass, had apparently been stolen from a senior soldier’s car while he was shopping at a supermarket.
The dossier reportedly reveals how the SAS and bomb disposal experts would be deployed in the event of a terrorist strike and how many helicopters are available to be scrambled.
It includes phone numbers for the UK's most important military figures such as the Defence Secretary, Chief of Defence Staff and Director of Special Forces.