Amid mounting concern over the risks to air passengers of deep vein thrombosis, the Government is setting up an Aviation Health Unit as recommended by a House of Lords report on air travel and health in 2000.

The Gatwick-based unit is being trumpeted as the first port of call for public enquiries and a body that would both investigate any potential health issues and identify the need for further research. However, it has yet to be staffed and will not be up and running before the summer.

'It is a start but a very small start. It's hardly going to address the issue properly,' said John Smith MP, who is chairman of the all-party group on travel-related DVT.

He said he was leading a parliamentary move to alter the 74-year-old Warsaw Convention that frees airlines from responsibility for health problems suffered on board. 'This legislation has to be changed,' he said.

The High Court ruled last autumn that families of DVT victims could not pursue a claim against airlines, who are only liable under the Warsaw Convention in the event of an accident.

John Smith's committee also said that anybody who had had hip or knee surgery should not consider flying long haul for at least three months after the operation. Dr Ander Cohen, an expert on DVT overseeing the research, said: 'Travelling for three hours or more results in a three-fold increase in risk if you have a prior risk factor.'

Risk factors include pregnancy, taking hormone replacement therapy or oral contraceptives, and recent surgery. Cohen said: 'There seems to be some extraordinary link between travel and blood clots.'

This year the UK government has given £1.2 million to World Health Organisation research on DVT and cabin pressure while airlines have committed to offer the study free seats on flights, and access to frequent flyers.

Due to the difficulty of diagnosis, there is uncertainty over the number of deaths caused by travel-related DVT.

Best estimates range from 200 to 1,500 per year. To put this in context, there were 3,599 deaths caused by motor vehicle traffic accidents in 2000.