Millions of people who travel to and from work every day on overcrowded trains could be putting their health at risk.

A preliminary study by researchers at the University of Nottingham has confirmed what many train passengers already know - they are suffering from dangerously high stress levels.

The finding has prompted the government-funded Rail Passenger Council to fund a much larger study to find out if so-called "cattle truck" train syndrome is putting lives at risk.

We don't really understand enough about the impact of overcrowding on trains on passengers' health Spokeswoman, Rail Passengers Council

Officials hope the research will boost their campaign to tackle overcrowding on trains across Britain.

Researchers are expected to measure passengers' heart rates, blood pressure and other possible symptoms of discomfort.

It will be one of the first attempts to scientifically measure the stress levels of commuters.

Health problems

Stress can cause a range of health problems, ranging from strain and back ache to a heart attack.

Surveys have consistently shown that British workers, particularly those living in London and the South-East, find commuting stressful.

A spokeswoman for the Rail Passengers Council said the preliminary study had revealed a lack of research in this area.

"We don't really understand enough about the impact of overcrowding on trains on passengers' health," she told BBC News Online.

"There hasn't been much work in this area. We hope this study will help us to find out more."

Council officials are currently drawing up a research brief. Research teams will then be invited to tender to carry out the study.

Last week, the Strategic Rail Authority warned that thousands of passengers are being forced into cramped carriages because firms find it cheaper to incur modest penalties than pay for longer trains.

It also follows a warning that the rail network in some parts of the country is congested and cannot cope with any more traffic.

One operator in the South-East recently told passengers it was considering turning carriages into "cattle trucks" by taking out seats to tackle overcrowding.

The Association of Train Operating Companies said more tracks and expanded stations are the only way to reduce overcrowding.

"There has been a 27% growth in passenger numbers over the past four years," a spokesman told BBC News Online.

"We have responded by increasing train services by 20%. We have done everything we can to run more trains. However, we have now reached the limit. Parts of the network are full.

"We need paths in the long-term. We could also expand services if we had more platforms at the major London stations. Ultimately we need more space."