London's largest estate agent is facing a multi-million pound payout after overcharging an Forest Hill landlord.

Dr Chris Townley, a lecturer at Kings' College London is taking legal action against Foxtons after being charged mark-up fees on work done at his property in Devonshire Road.

Dr Townley discovered this after being invoiced £550 by the letting agent for installing two exterior lights in his property in April 2013.

He said: "The contractor was sent round to fix the external security light behind a plant.

"They drilled a hole through the front wall into the sitting room."

He added: "They affixed a plug to this and then simply plugged it into the socket rather than attaching it to the mains."

After contacting the contractors, Dr Townley discovered they had charged £412.50 for the work and had added a commission fee for Foxtons, which was not set out in the invoice.

Foxtons eventually admitted they had taken a huge commission on all work done at Dr Townley's property by a contractor, totalling 38 commissions and about £1,900.

In many cases the commission was as much as 33 per cent of the bill.

In Dr Townley's tenancy agreement it stated that Foxtons may retain commissions from third parties, but the amount is not specified.

He said: "I was incredulous when I found out Foxtons took commissions from anyone working on my property without my consent. I felt betrayed."

Dr Townley also believes Foxtons took out the most expensive contractor they could find so they could earn more commission.

He added: "I compared the Foxtons prices with others I'd found on the internet and even when I took out the commission the prices were two to three times more expensive than what I found in five minutes."

Law firm Leigh Day have taken Dr Townley's case on a no win no fee basis and are encouraging other landlords to come forward, which could end up with Foxtons paying out as much as £42 million.

Leigh Day solicitor Chris Haan said: "We believe these charges to landlords are unlawful as they are not sufficiently disclosed, so the landlords cannot give fully informed consent to them.

"These kind of practices may be widespread in the lettings industry and it needs to stop."

A Foxtons spokesman said: "We are incredibly disappointed to hear when any customer is dissatisfied with the service they received, however, as a legal dispute we are not in a position to comment on the specifics of this case.

"We are satisfied our fees are clearly laid out within our terms and conditions and that approvals are obtained from our landlords before works commence on their property."