Smart casual has become the dress code du jour among the business community as the ties are loosened

UBS HAS just a "few general rules" for its dress policy: "There are no flip-flops, no shorts and shirts have to have collars. We are not exactly dress-down, but we are more relaxed than many City institutions," according to a spokesman at the Swiss investment bank.

"People don't have to wear suits and have freedom within pretty obvious guidelines. If you are meeting clients, you dress appropriately."

As the mercury heads north, the thoughts of many in the business community turn to packed, pressure-cooker trains, shirts stuck to backs, constricting ties and just how broadly they can interpret the phrase "smart casual".

Shorts at work remain a no-no in almost any part of the white-collar economy. The good news, though, is that companies even in traditionally buttoned-up sectors are embracing a kind of smart-casual liberation.

In adland, it seems, there is little in the way of formal rules. "We don't have a dress policy at all," says Farah Ramzan Golant, chief executive of UK advertising agency Abbott Mead Vickers BBDO. "People are allowed to express their identity through what they wear. We wouldn't want to start dictating skirt lengths, as some law firms do."

However, she notes, there are unspoken rules.

"The main thing is to look sensible in front of clients. I would not expect people to wear shorts or flip-flops but I would never inspect what people are wearing. Everyone is a brand and every brand expresses itself visually, but it is less about being correct and more about being communicative."

This attitude is to be welcomed, says Brendan Barber, general secretary of the Trades Union Congress. "When the weather is hot, staff need to stay cool, and sensible employers understand that it makes sense to allow employees to dress down for summer."

Barber adds that while he is not advocating bikinis and bare chests, "allowing employees to discard their tights, ties and jackets will avoid them feeling faint from the heat at their desks and allow staff to carry on working productively . . . a cool smart-casual approach is needed".

Cary Cooper, a professor at Lancaster University management school, takes a similar line.

"If you're not client-facing in offices, I do not see why you cannot wear smart casual - you can always take a tie along and put it on if you need it. We should lighten up in the office."

Yet, he adds, there is still a certain image that goes with business - whatever they wear, staff should feel as if they are at work. This, Ramzan Golant says, is why she frowns on shorts - not because she dislikes them, but because of the message they send out. "Shorts signal that you are off duty, that you are on holiday. That is why they belong on the beach and not in the workplace."

Dress also varies with sector. Business casual still has some way to go in politics and the law: when was the last time you saw a male member of parliament in the House of Commons without a tie?

In the technology sector, policies are more relaxed. While a spokeswoman at Bebo, the social networking site, rules out beachwear, "smart tailored shorts would work", she says. Visitors to MySpace's London office on a summer's afternoon, meanwhile, will encounter anything from maxi-dresses and dungarees to the latest Manolo Blahniks.

Perhaps the greatest shift in office attire, however, has been in financial services. Once a buttoned-up industry, its change in attitudes is now apparent, from the open-necked hedge fund managers of Mayfair to the big banks.

"If you walk around the City, you will see people looking smart, but shirt-and-jacket smart, not shirt and tie," Cooper says.

While business casual is a surprisingly broad church, at its limits would be Agent Provocateur, a British lingerie company.

Sarah Schotton, senior designer, describes the policy as "business sexy". "We're supposed to dress smartly and sexily - like the brand - so that means high heels, make-up and dressing up, and no jeans and no trainers."

Would Agent Provocateur countenance shorts if the weather got hot enough? According to Schotton: "Well, I suppose you might get away with a pair of sexy hotpants - but only if your bottom was up to it."

- (Financial Times service)