Kings College Hospital NHS Foundation Trust has a contract with a firm called Caring For You (CFY), which provides non-emergency ambulances.
But it claims that drivers from the firm are flouting guidelines on where and for how long drivers are allowed to park.
King’s College Trust says it must now resort to clamping CFY ambulances breaking the rules so other vehicles, including emergency ambulances, can move freely.
A trust spokesperson said: ‘Unfortunately, some drivers have not been following these guidelines and have been leaving vehicles in drop-off bays for longer than the permitted time, as well as in no parking areas.
‘Despite frequently raising this issue with CFY, the problem has persisted. King’s therefore has reluctantly taken the decision to start clamping CFY vehicles when drivers do not follow the guidelines.’