Estate agents have come under fire again following a television expose last week. The skulduggery behind one of their many dodgy practices is unveiled. Whatever your view on estate agents their signboards are a facet of modern life.

And these days, it is not just a surge in the market which prompts them to get fruitful and multiply: a significant proportion of the boards on our streets are actually planted there illegally.

It is one of the many dubious practices highlighted in the BBC's Whistleblower documentary on estate agents last week.

So-called "flyboarding" is not a new phenomenon, but every time measures are taken to stem this practice, it pops up again somewhere else. In its mildest form, it occurs when an agency "forgets" to take down a board, once the sale or letting has been completed.

But sometimes agencies set out to saturate an area with boards in order to drum up business, irrespective of how many instructions they actually have. Usually, these are in neutral settings where they'll be seen by passing traffic, but not reported by local residents. But agencies sometimes make mistakes, most famously in the case of Labour's former spin-doctor Alastair Campbell, who woke to find a "For Sale" sign outside his house. The stars of Whistleblower, Foxtons (for it was one of theirs) hastily apologised, removed it and sent Mr Campbell flowers.

Recently, the Royal Borough of Kensington and Chelsea (RBKC), which has some of the most valuable properties in the country as well as conservation zones, has taken stringent measures against flyboarders; and its efforts are being echoed by other London boroughs.

The last crackdown by the borough was in 2003, and resulted in 10 agencies being fined for the offence. At the time, the fines were small, with Foxtons scooping the biggest pot, with a trifling total of £750. But now, the borough has come down more heavily. In December, magistrates fined estate agency Chard £18,000 for flyboarding, after having been tipped off by… Foxtons.

Councillor Fiona Buxton, cabinet member for environmental health at RBKC, said: "Flyboarding gives a misleading impression of the amount of business an agent is doing in an area, and is intended to affect the choices of potential sellers when selecting an agent to market their property. We will not tolerate this kind of behaviour, and I hope this successful prosecution sends out a warning to other estate agents who may be considering flyboarding."

Barry Manners, a managing partner of Chard (which also featured in Whistleblower), points out that his company's offence was one of omission - forgetting to remove a board from a property where they'd had a genuine instruction - rather than deliberate flyboarding. He also says he supports a zero tolerance attitude, provided it applies evenly, creating a fair playing field.

"Flyboarding used to be very widespread here. Any railings in London seemed like fair game. And it was very competitive. There was a time four or five years ago when we'd lose 100 of our boards every night. A sub-contractor for one of our rivals - we'd successfully taken a lot of their business - used to go round with a van at between three and five in the morning, chopping them down and bundling them in the back. It was so organised that the police even became involved, although nobody was taken to court."

A former flyboarder for a well-known agency, now a cabbie, gave me an insight into the practice. Of 150 boards a week he put up, he estimated that up to 50 per cent were illegal. "It's not rocket science. You have to choose a site where the residents are not going to challenge the board," he continued. "A block of flats or a conversion is ideal, somewhere where people don't talk to each other. Don't put it outside any block that's got a sense of community - they'll soon realise nobody is selling and take it down."

His usual technique was to put the board somewhere relatively inaccessible, somewhere you might need a stepladder to pull it down again. That way only another estate agent's contractor, or the council, would remove it. And it would be on a road with plenty of traffic; there's no point in wasting effort erecting a fake board in a cul-de-sac.

A popular option was to place it on a main street that could then claim to refer to a genuine instruction up a side-road, and then to "accidentally" leave it there when the property sold. All these locations were, he said, agreed with the client, because every estate agency has to maintain a list detailing all its boards. "In fact, there were usually two lists. One for the eyes of the Trading Standards people, should they want to do an audit, and one for the agent's own use." And it would be up to the sub-contractor to make sure that the board, once up, stayed put. Some contractors did, he admit, go round pulling others' boards down, although it wasn't something he had been prepared to do.

Paying a fee of £5 to install a board is a cost-effective way to advertise your business. But it can turn into a heavy burden if you are unlucky enough to be a new agency targeted by the competition.

For Colette Brown, who last year started Property Garden in Chiswick, west London, board wars have proved a logistical and costly nightmare. She says the Whistleblower team filmed her boards being taken down, although this footage was not broadcast.

"We're a small agency and we only have up to 25 (boards) up at any one time. One weekend last August, 18 of them disappeared in one go. I know why it happens - we undercut our competitors by 50 per cent on commission. So our boards get taken out." Colette offered a £500 reward for information, but as yet has received no response.

Barry Manners would be happy to see all boards banned. "Why do we need them? These days we've got the internet. Does anyone really go round looking for boards to find a property? Besides, it is easy to track a board back to an agent's website and see whether it relates to a genuine property, so I think the net will eventually do away with flyboarding altogether."

In the meantime, the Borough of Hounslow is currently highlighting its own crackdown. Having audited 144 "For Sale" and "To Let" signs, Hounslow Trading Standards officers contacted the relevant agencies and discovered that 52 of them (36 per cent) were illegally displayed.

One "For Sale" sign was on an electricity substation, while another "Let By" sign turned out to have been on display there for about four years. Forty-two of the offending boards have been removed, and letters have been sent out to the remainder, warning of impending prosecution.

To test the effectiveness of this Hounslow purge, the status of 10 boards lined up along a main road near my home were checked. Of the 10, only four had copper-bottomed reasons for their existence, and could be matched up to properties on relevant estate agents' website. A fifth was sponsoring a school fair, and a sixth, high on the wall of a primary school, was sponsoring something, but didn't bother to stipulate what.

Numbers seven, eight and nine claimed that properties I was inquiring about had "gone - what is it you are looking for?" (They have two weeks' leeway before the board's presence becomes an offence). And number 10, after vainly trying to attribute the board to a property in the region, could offer no explanation as to why it was there.

Copyright © 2005 First Mortgage Trust