Property agents are overcharging leasehold owners of flats by taking out unnecessarily expensive building insurance on their behalf and pocketing a share of the commission, according to an inquiry.

An investigation into property fees by the Royal Institution of Chartered Surveyors (RICS) has found evidence that insurance brokers and agents or freeholders routinely split commission earned from insurance policies, which are paid for by leaseholders.

There is growing concern that leaseholders are unaware that up to 30 per cent of the sum they have been asked to pay is commission or that the policy is uncompetitive, because these details are not disclosed on the lease.

The practice has left millions of leaseholders across the UK unknowingly paying a commission of £90 a year on a typical £300-a-year building insurance policy to agents and insurance brokers, industry experts said.

Roger Southam, chairman of Chainbow, a property management company, said: “It is mind-blowing how many residential buildings have inflated building insurance premiums. The reason why no one bothers to highlight them is usually that there is a lot of money to be made. If an insurer says he can charge a certain amount in premiums but will give the freeholder or agent a percentage of the sum for appointing a particular insurance company, both parties win and the leaseholder is none the wiser.”

A spokesman for RICS said that it was aware of “unwelcome practice in this area”.

The problem is one of many areas of concern identified by RICS in the unregulated estate agency sector. It will also investigate service charges for leasehold property, commission on letting renewals, commission on Home Information Packs and valuation fees before publishing a report at the end of the year.

Fears were also raised that home-sellers may not know how much commission they are being charged by a Home Information Pack provider.

The greater scrutiny of property agents’ fees comes shortly after the High Court ruled that commission that landlords were charged by Foxtons, the estate agency, when a tenant renewed a contract or bought the property was unfair and represented a trap.

Although agents are unregulated, insurance brokers are regulated by the Financial Services Authority.

The British Insurance Brokers Association, which has been asked to respond to RICS, said: “Insurance brokers have an agency relationship with either the property managing agent or property owner and have duties to them under the laws of agency.”