One particularly annoyed viewer who contacted Watchdog about this problem is Ian Bolderston. In the space of two and a half years, his BMW dealer advised him to replace six wheels. They found that one wheel had seven cracks on it. With new tyres and associated costs, replacing the six wheels set him back £4,000.

David Morgan is another disgruntled BMW driver. He's had to replace three wheels at a total cost of £1,200.

BMW tested Ian and David's nine wheels and decided that they had all been damaged by a road impact - such as hitting a pothole. But the drivers said they just drove normally, and weren't to blame.

"I asked BMW and the first thing they told me was perhaps you've hit a pothole, you've hit something," Ian said. "I'm a normal driver with three kids. I'm the wrong side of 40 to be racing around. As far as I'm concerned it's BMW's fault. They need to get to the bottom of the problem. It's not potholes as far as I'm concerned. I've driven normally. There's something fundamentally wrong with these wheels."

David reflected on the fact that that all the cracks appear on the inside wheel rims, and almost always affect the rear alloys. "You hit a pothole with a front wheel. I've had no front wheels damaged. There's more weight over the front of the car than the rear so I'm astounded as why the fronts haven't cracked," he said.

The problem centres on a particular combination of wheel and tyre. The cracking seems to only affect BMW 19-inch rear alloys, coupled with runflat tyres. The tyres are a safety feature designed so that they can still be driven on when they're flat - the idea is that if you get a puncture on the motorway at speed, with runflats you won't lose control and can continue driving to a garage where the problem can be sorted.

Watchdog researchers called BMW dealers posing as customers. These confirmed, in many cases, that BMW expected the cause to be potholes. One dealer thought the damage happened because the low-profile tyres may have been slightly underinflated meaning that hitting potholes at speed could crack the wheels.

Some of the BMW dealers seemed to wonder whether these flashy wheels are suitable for British roads. One dealer said, "The wheels and tyres aren't designed for the roads really as they are in Surrey at the moment." A second said, "They use the same wheel and the same tyres all over the world, and I think it's due to our road surfaces. We're the only country that has a problem with it."

Potholes and other road surface hazards certainly have an impact on drivers' pockets. The AA reckons there are 1.5 million potholes in UK roads, and it's estimated that they cause billions of pounds of damage to car drive shafts and suspensions every year.

But with at least two damaged BMW wheels, potholes don't seem to have caused the cracks. Self-confessed BMW fan, Dr Aamer Khan, was outraged when he was told he would have to pay to have three wheels on his car replaced while the car was still under warranty. So he decided to pay £1,700 for international experts MIRA to investigate. Their report strongly suggested that one wheel had a manufacturing defect; it implied a second wheel had the same problem, and it decided that a third wheel that a BMW dealer said needed replacing, was not damaged.

Aamer started legal action and 18 months later, BMW paid him £7,000 as a goodwill gesture. "However, they were not taking responsibility or liability for the damage to the wheels hence they were still saying there is no problem with the wheels, which I do believe is incorrect," Aamer said.

Watchdog wanted to try to get to the bottom of the problem, so we collected nine damaged wheels and asked engineering expert Mark Brown to investigate. He's been in the business for 30 years and had never seen this type of problem before.

BMW dealers had decided that all nine wheels had suffered impact damage. But Mark Brown could find no visual evidence of this. "There's just no evidence of pothole damage. There's no evidence of any impact damage of any significance around the inner rims of the wheels, the places where the wheels are cracked," he said. He added it was "unfair" for BMW to refuse to replace the wheels under warranty.

And he questioned the suitability of the wheel design. "It may well be that driving over speed humps and potholes is causing the cracks but the wheels should be man enough for the job. They should be able to withstand that sort of use," he added.

Mark believes BMW should take action to deal with the problem with these particular wheels. "I think they should seriously look at the design of the wheel and seriously consider a safety recall," he said.

BMW statement:

"We put customer service and satisfaction at the top of our agenda and so take any concerns from our owners very seriously. We have been contacted by a number of customers who have experienced cracks in alloy wheels. Every one of these has been investigated by our specialist technicians and in every case the wheels concerned have shown signs of impact damage, which have resulted in the cracking. This is likely to have been caused by driving at inappropriate speeds over speed humps or through potholes. Equally, curb impacts have been to blame in some cases.

"We have received calls from around 100 customers out of the total 1.3 million BMWs on UK roads from owners reporting problems with cracks in alloy wheel. Every one of these has been investigated by our technical experts and in every case the wheels concerned have shown signs of impact damage, which have resulted in the cracking.

"This is likely to have been caused by driving at inappropriate speeds over speed humps or through potholes. Equally, curb impacts have been to blame in some cases.

"BMW takes great pride in developing the highest quality equipment for all its cars, with some of the most rigorous test standards in the whole automotive industry. Wheels are tested on all manner of different surfaces before they are approved for use. However, no wheel is indestructible and if a car is driven too fast over a speedhump or through a pothole, damage can be done.

"A road tyre plays an integral part of any vehicle's ride and comfort systems. If the tyre pressures are not correctly maintained the tyre will not have the optimum absorbing properties allowing energy to pass straight into the wheel/suspension components, which in turn, if driven inappropriately, could damage both.

"If the wheel is above the tolerance then the wheel has suffered an impact. As a result of poorly maintained roads, it is well publicised that on average across the UK, six per cent of all vehicles will suffer pothole damage in a 12-month period.

"In some instances no visible damage will be apparent. As mentioned above, the tyre can absorb energy to a certain degree. Once beyond this threshold, the excessive loading is transmitted to the wheel and may cause cracking, although the tyre may protect it from obvious visible damage apart from the cracks.

"Under the terms and conditions of the warranty, BMW will always pay for defects that are attributed to a manufacturing defect. However, our investigation of these cases has shown no signs of such a defect, but rather that the damage is due to an impact. In this set of circumstances there is no case to cover the repair costs under warranty. If a wheel rim is presented with cracks, a full technical evaluation is made in every case. If the wheel is found to be deflected from a perfect circle by 0.3 mm or more this is evidence that an impact has taken place and this has been the cause of the cracks. In this case the customer would be responsible for the repair. If the wheel is deflected by less than 0.3 mm and yet is cracked, then it will be replaced under warranty.

"If any BMW customer is concerned about this, or any other issue, they should not hesitate to contact our customer helpline on 0800 777 129."

THE SOCIETY OF MOTOR MANUFACTURERS AND TRADERS (SMMT) TOLD WATCHDOG:

"Motor Codes Limited is the body responsible for the self regulation of the motor industry through consumer codes of practice. Supported by government and the motor industry they aim to raise standards and increase consumer protection for motorists. Their Codes cover garages, vehicle manufacturers and extended warranty administrators.
They can be contacted on 0800 692 0825 or at http://www.motorindustrycodes.co.uk."