The Liberal Democrats are calling for a change in the law so that people with chronic illnesses like asthma or cystic fibrosis do not pay for prescription medicines.

Backbench MP Paul Marsden presented a Ten Minute Rule Bill in the Commons on Wednesday which would exempt these patients from charges.

However, with the current session of parliament due to end in just a few weeks the Bill has little prospect of becoming law even if it is supported by MPs.

Our policy is to give priority to helping people who may have difficulty in paying charges

Department of Health spokeswoman

Mr Marsden or another MP would have to reintroduce the Bill when the new session of parliament begins in the autumn.

Nevertheless, the Lib Dems hope the move will highlight what they have described as an unfair and illogical system.

Exemption policy

At present, some patients with chronic illness are exempt from patient charges if their condition is on a Department of Health list, originally drawn up in 1968, or if their income is below a certain level.

Patients with chronic illness can also pay �89 up front to cover the costs of their prescriptions over a year.

However, a survey by the National Association of Citizen's Advice Bureaux has found that 27% of those who could have benefited from so-called season tickets did not even know of their existence

The Lib Dems said the finding strengthened their case for exempting patients with chronic illnesses from prescription charges.

Mr Marsden will introduce his Bill on Wednesday

Mr Marsden said: "My Bill aims to deliver justice for thousands of sick people forced to buy their medicines on prescription.

"It's time Labour realised that the current system is unfair and forces less well off patients to choose which drugs they can afford to buy."

The party's health spokesman Dr Evan Harris added: "These charges are unfair and deter many patients from getting the treatment they need.

"People with long-term illnesses should not be left with the burden of prescription charges for their whole lives."

The party estimates that the cost of scrapping prescription charges for these patients would be in the region of �83m each year.

A spokeswoman for the Department of Health said: "Our policy is to give priority to helping people who may have difficulty in paying charges, rather than extending the exemption arrangements to people with other medical conditions including cystic fibrosis.

"A review of prescription charges was undertaken in 1998 as part of the government's comprehensive spending review.

"It was noted that there was no consensus on what additional conditions might be included in any revised list of medical exemptions, or how distinctions could be drawn between one condition and another - it would not be right to consider one group in isolation".