Some even have their pay cut every time they spend a penny.

And more than one out of every four workers say they cross their legs because of the poor state of factory and office facilities.

But on Friday the Trades Union Congress (TUC) is launching a campaign, called Gotta Go, to make bosses treat staff with greater respect.

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UK health and safety regulations state employers must provide enough suitable, clean, accessible, adequately ventilated and well-lit lavatories to allow everyone at work to use them without undue delay.

But a legal loophole means employees do not have the right to use them at their convenience.

Staff at a factory in Dumfriesshire have to use an electronic smart card to excuse themselves, according to the TUC survey.

And one worker was docked an hour's wages, �5.28, during the course of a single week.

If they trusted and respected their staff, and treated them as adults, not naughty children, they would end up with a healthier, better motivated, more productive workforce TUC

The TUC is calling on the government to make it clear workers can use the loo whenever they have the urge - except where that could endanger their colleagues.

It hopes the practice will become "bog standard" and stop bosses spying on staff or keeping notes on how frequently they go and how long they take.

At one workplace, staff said part of the toilet door was cut off so managers could check they were not smoking.

Elsewhere a pregnant night-shift worker at a factory in Southampton had to be escorted by a security guard across a badly lit car park to the nearest women's toilet.

TUC general secretary elect Brendan Barber said: "It is incredible that in the 21st century workers are still being penalised for going to the loo.

"Employers clinging to Dickensian bathroom-break policies should understand that if they trusted and respected their staff, and treated them as adults, not naughty children, they would end up with a healthier, better motivated, more productive workforce."