Health staff will not ask individuals who turn up at one of the HSE's 45 swine flu vaccination clinics around the country to prove that they are in one of the 'at-risk' categories for the disease.

This raises the possibility that individuals seeking to skip the queue for the vaccine will be able to do so by simply stating that they have one of the illnesses which would qualify them for priority access to the vaccine.

However, the HSE is urging members of the public to respect the decision to prioritise the vaccination of people in these groups.

There are between 400,000 and 500,000 patients in the 'at-risk' group, including those with chronic heart and lung conditions and women more than 14 weeks pregnant.

About 200,000 doses of swine flu vaccine have already been sent to GPs, who will get the same amount again over the coming weeks. But a further 100,000 doses will be available at the HSE clinics as the vaccination programme gets underway from tomorrow. A HSE spokeswoman stressed that the majority of patients will get the vaccination from their GP.

Over 2,300 participating GPs have been receiving supplies of the vaccine in the last week and half, she said.

"Anyone in the at-risk categories whose GP is not participating can be vaccinated at a HSE clinic. They will not be asked to 'prove' that they fall into an at-risk category but the HSE would like to stress that it is important that this group is vaccinated first and that the rest of the population will be vaccinated at a later date," she said. "The public is being asked to respect this decision, which was made based on expert clinical advice.

"There is going to be a certain degree of trust involved. The wider public need to realise why people are being prioritised… if you are not in one of these groups and you are out there trying to get a vaccine you are taking a vaccine from someone who really needs it."

The spokeswoman also confirmed that cost was a factor in the decision to limit the opening hours for the clinics to weekday office hours.

She added that the HSE has put in place a strategy to double the number of "critical care" beds in the hospital system to deal with swine flu patients should this prove necessary. There are currently 289 such beds available in the system.

The state's chief medical officer Dr Tony Holohan last week said there was increased pressure on hospitals, particularly children's hospitals, which have had to cancel some elective surgery to cope.