Most commonly a faulty wheel sensor but there are lots of things it could be.
Check your tyres are all the same size/aspect ratio/pressure.
Best to get the fault codes read. Phone around for the best price, lots of garages have the diagnostic equipment.
I wouldn't pay more than £35 for the check. Some places will refund the test cost if you give them the repair job so factor that into the cost.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.