The company brought in a firm of security experts to sweep the premises after a burglary at its Liverpool branch two months ago in which computer disks containing thousands of patients’ details were stolen.

The subsequent discovery of the listening device two weeks ago has led to fears that the private clinic may be the victim of a dirty tricks campaign.

At Greenwich CID, Detective Constable Nicholas Swindells said: “We are keeping an open mind about motive but it could be that someone is targeting this company because of the wider political issues involved. We will take the investigation as far as we can.”

The Department of Health has emphasised that it does not support single vaccinations for measles, mumps and rubella. The government has mounted campaigns to persuade parents the MMR jab is safe after some research linked it to autism and bowel disorders in children.

Falling uptake of the triple vaccine around the country have led to outbreaks of measles among children. So far no child has died but more local epidemics are expected.

The bug was found by Stuart Wright, of Spectrum, a Kent-based security firm and was passed to police by Direct Health 2000 for analysis. Wright said it was a simple FM device with a transmitting range of about 300 yards and a battery life of up to 10 weeks.

“The person who planted this device would have been able to listen in from a neighbouring building or vehicle or may have had an automatic recording machine hidden nearby,” he said. It was planted in the area where staff take calls from patients and offer advice. The company has given single jabs to at least 20,000 children.

Sarah Dean, Direct Health 2000’s managing director, asked for the sweep after it was noticed that light fittings and ceilings at the Liverpool office had been interfered with during the burglary. At the time, police believed it might have been an attempt to find out whether Tony Blair’s son Leo — or the children of other prominent people — had been given single jabs rather than MMR.

Last week Merseyside police renewed their inquiries into the Liverpool burglary and are liaising with the Metropolitan police. One theory is that the bug may have been placed by an insider or someone with access to the London office as there has been no evidence of a break-in.

Officers are also investigating how a consignment of potentially dangerous mumps vaccine was sent to one of Direct Health 2000’s suppliers. The vaccine sent by a German distributor turned out to be of the Urabe strain, banned in Britain since 1992 as it can cause viral meningitis in babies.

“Our worry is that if the vaccine had been used unintentionally by us, or even been brought on to our premises, it could have been used as an excuse to shut us down,” said a Direct Health 2000 spokesman.