The office Christmas party is a chance to let your hair down but there's a danger of waking up with something a lot nastier than a hangover the next day.

One in five say they've had a sexual encounter with a co-worker at the annual bash, according to a new survey.

And one in ten admit they've caught a sexually transmitted infection (STI), such as chlamydia and gonorrhea.

Worryingly, the fun could turn into a nightmare and result in your colleagues filing a complaint with managers or even land you with a law suit, the research also suggests.

It found 13 per cent would tell a manager or another colleague if they caught an STI from a co-worker, eight per cent would tell HR and four per cent would speak to someone in their company’s legal team.

It raises an alarm that people could unknowingly end up in trouble, because many people with chlamydia and gonorrhea – known as the 'silent' infections – do not have symptoms and are oblivious they have them.

One in ten people admit they've caught an sexually transmitted infection (STI) from a colleague, research suggests (stock image)

On the other hand, a shocking 13 per cent of people confessed they wouldn’t be able to face telling the colleague they slept with if they caught an STI.

Left untreated, chlamydia and gonorrhea could damage fertility for men and women.

The results of the poll of 2,000 UK office workers were released by independent health platform, Patient.info, supported by the sexual health charity FPA to highlight the risk of STIs during the party season.

Men are 30 per cent more likely than women to get intimate and are 40 per cent more likely to have caught an infection.

The research also revealed that 18-24 year olds are nearly six times more likely to have caught an STI from a colleague than 45-54 year olds.

Yet 55-65 year olds are the most likely group to have hooked up with a colleague, the study also found.

Worrying attitudes

It revealed almost one in three workers wouldn't be worried about catching chlamydia – the UKs most common STI.

If left untreated, the infection can spread to other parts of your body and lead to long-term health problems, such as pelvic inflammatory disease (PID), according to NHS Choices.

It can also cause epididymo-orchitis (inflammation of the testicles), and sometimes reactive arthritis.

Meanwhile a quarter of people reported feeling complacent about catching HIV.

Dr Sarah Jarvis, GP and clinical director of Patient.info, said: 'STIs are hugely prevalent in the UK, with more than 400,000 new cases being diagnosed each year.

'It's important that people know how to take the right precautions and limit their risks, especially with party season ahead.'