Police said that last Thursday, IRA members are believed to have broken into a compound in Trafford Park, Manchester, and set fire to an Iveco truck, identical to the one stolen from a haulage yard near Newcastle-under- Lyme in Staffordshire in March, which was used in the bombing.
The tipper truck was fitted with the new number plates and resprayed in the same dark blue colour as the Manchester vehicle. Police believe the extensive precuations were taken to ensure that any casual computer check on the vehicle would show the registration number as genuine.
It is assumed that the Manchester lorry was destroyed to avoid any chance of the two being seen on the roads at the same time between last Thursday and Saturday.
Anti-Terrorist Squad detectives are trying urgently to build up a picture of the movements of the eight- wheeled vehicle between its theft on 20 March and last Saturday morning.
'The IRA must have access to large premises, such as a shed, barn or yard, to store, respray and load the vehicle with the one-ton bomb of fertiliser chemicals,' a police source said.
Yesterday detectives continued to question about 10 people, including women, over the Bishopsgate bomb and the two bombs planted in hijacked taxis on the same evening.
Senior police officers said that they would be making a formal approach to the Home Office to seek an amendment to the Police and Criminal Evidence Act to allow them to carry out more frequent road checks. Chief constables believe the existing checks are legally justifiable only when there is direct evidence of a threat - rather than of the general existence of an IRA campaign.
John Burrows, president of the Association of Chief Police Officers, said after the association's quarterly council meeting in the City that police did not want permanent checkpoints. 'The last thing we want to do is alienate public opinion.'
Mr Burrows said provincial chief constables had expressed confidence in the current system of investigating incidents, where local detectives work with the anti-terrorist branch overseen by an Acpo sub-committee. He added that 'good working relationships' had been established with the security service over intelligence gathering.
The meeting agreed to set up a small regional anti-terrorist office in the North-west, where there have been many IRA attacks, staffed by local detectives trained by the Metropolitan Police Anti-Terrorist Branch.
Both Scotland Yard and City of London Police declined yesterday to elaborate on the disclosure that a general national alert of IRA activity was issued on Friday. City of London police established an armed road check in the Bishopsgate area on Friday night. Sources said such alerts were common.
Yesterday, the IRA released a statement in Dublin admitting responsibility for the two mini-cab bombs.
'These latest attacks underline both the ability and the determination of our volunteers to breach whatever level of security the British authorities are capable of mounting.'