Direct Health 2000, in Liverpool, offers separate measles, mumps and rubella vaccinations.

But of the 3,332 children who were signed up to the course of treatment in 2001, only 150 have completed the set one year later.

The clinic said Department of Health restrictions on importing the mumps vaccine were responsible for the delays.

We have chosen to go down this route because it's the safest route for our child

Tracy Lashford, parent

Direct Health Chief Executive Sarah Dean said: "We are having a severe problem with the mumps supply line.

"We have found that there are restrictions being placed onto how many vaccines, or mumps vaccines particularly are allowed into the country at any one time."

The Department of Health limits imports to 25 doses of each order of the vaccine per day.

It said the vaccine was not intended as a mass alternative to the MMR jab - which has been linked with autism.

When clinics began offering separate injections, the department said it was worried some parents would not complete the full course.

The clinic attracts parents worried about MMR

But parents at the clinic said they were growing frustrated with the delay.

One - Tracy Lashford - said: "We have chosen to go down this route because it's the safest route for our child and we can't see any reason why we can't be allowed to do that."

She added she was worried she might have to wait several months for her daughter, Emily, to complete the set of vaccinations.

But she said she was determined to see the course through.

The clinic said it informed each child's GP once they had been vaccinated, so that they could be monitored.

The North West Office of Public Health said it believed the triple MMR vaccine still remained the safest option available to parents.