What's the first thing you did when you arrived in the office this morning? Before even checking your email or launching into a 10-minute rant on how two-day weekends just aren't long enough, you probably made yourself a cup of tea.

Around 165m cups of tea will be consumed today in the UK, accounting for 40% of the nation's fluid intake. Given our long working-hours culture, that means an awful lot of tea is drunk in the office. And according to dietician Dr Carrie Ruxton, who has studied the impact on health of the consumption of black tea, this is no bad thing.

"Tea contains polyphenols that mop up the free radicals that can damage blood cells," says Ruxton (favourite tea: Assam, lots of milk, no sugar). "Three cups of tea a day is associated with an 11% decrease in the risk of heart attack. That's a pretty significant statistic. And tea flavonoids can also help control surges in blood pressure."

Leave the colleague yabbering about how tea dehydrates you to her latest health fad, and get a brew on. Both tea and coffee are beneficial to hydration. "You'd need to drink about five cups of tea within an hour for it to have a diuretic effect," explains Ruxton. "Drinking tea is a legitimate way of getting the recommended one-and-a-half to two litres of fluid a day." Tea might even have a positive impact on dental health if it has been grown in a fluoridated area or is made with fluoridated water, she says. It also reduces levels of the bacteria that cause tooth decay.

Coffee, like tea, is a source of polyphenols, but contains caffeine levels of around 75mg to 100mg a cup, compared with 40mg for tea. A moderate intake of caffeine can improve your work as it increases concentration and alertness - you would need to consume around 500mg of caffeine a day to risk side-effects such as headaches. That could equate to as few as five cups of coffee, but it's more than 12 cups of tea (at which point bladder-ache would presumably be as big an issue).

Stuart Payne (favourite tea: PG Tips, milk, one sugar) runs celebrated website NiceCupOfTeaAndASitDown.com. "Making a cup of tea is that excuse you need to get away from your computer screen and have a break," he says. "I think that wise employers recognise that people aren't actually wasting their time at work when they go off for a cup of tea. Whatever hectic stuff is going on around you goes on hold for a minute while you go through what is a familiar and comforting routine."

People have been doing things with tea (if not tea bags, which weren't invented until 1908) in Britain since the 17th century. Both black and green tea derive from the tea bush, camellia sinensis. The drink's association with the workplace dates from the mid-18th century, when benevolent employers would provide a their workers with a separate room in which they would be allowed to drink tea twice a day.

"By the end of the 19th century, tea had become our national beverage because everybody had come to see it as a drink they could rely on when they were tired or stressed," says Jane Pettigrew (favourite tea: jasmine, taken at the Dorchester), who has written 11 books on the subject. "In 1916, the Ministry of Munitions health committee even stated in a booklet on working hours that an 'opportunity for tea is regarded as beneficial both to health and output.'"

Tea has now become so central to the modern workplace that Stuart Payne believes the corporate recruitment process ought to take a candidate's tea-making prowess into consideration. And he's only half-joking. "The first time you go into the kitchen with a new employee and they do something dreadful like put the milk in with the tea bag before they put the water in, you think, 'I'm going to have to re-evaluate everything I thought about you. Because you really are out of order making the tea like that.'"

Perhaps over-zealous milk-pourers should take note.

One lump or two?

· Few aspects of the working day are as fraught with office politics as making the tea. If you remember one thing, it should be this: don't ever use another person's special mug. Doing so, is an office faux pas akin to head-butting the boss.

· It's more socially acceptable to keel over from dehydration than it is to use someone else's milk.

· Some of your colleagues will resolutely fail to ever get their round in. This is an immutable fact of office life about which nothing can be done.

· If a mug with a tea bag and spoon in it has been abandoned next to a just-boiled kettle, you're within your rights to use the water (ignore this rule if it's the managing director's mug).

· You don't always have to undertake a massive tea-run: nipping off and making yourself a beverage is perfectly reasonable, according to Stuart Payne, author of Nice Cup of Tea and a Sit Down. "Tea rotas can be a real burden," he says. "They're always driven by some basically lazy person who sits at the back and goes, 'Aaah, whose turn is it to put the kettle on?' And you think to yourself, 'You were probably spoilt by your mother, weren't you?'"