a gross plated weight of more than 3.5 tonnes or
if it has no gross plated weight, but an unladen weight of more than 1525kg.
An operator's licence is necessary even if you use the vehicle for only a short period of time, even just one day.

If you decide to use a vehicle without an operator's licence when you should have one, you risk having it impounded by the Vehicle & Operator Services Agency under new regulations*

The impounded vehicle will be either sold or scrapped after 21 days unless an application has been made to the local Traffic Commissioner for its release and the application is subsequently successful .

The only grounds for making an appeal are:

at the time VOSA impounded the vehicle, the person using it had an operator's licence
at the time VOSA impounded the vehicle, it's use was exempt from O licensing
the vehicle was being used illegally without the owner's knowledge
Unsuccessful appeals will result in the vehicle being either sold or scrapped. There is a further right to appeal to the Transport Tribunal but the grounds are exactly the same.

If you think you might need an operator licence - contact your local Traffic Area Office for advice before using the vehicle.