Charles's favourite architect fined for knocking down Nash gate houses

Quinlan Terry, 70, pleaded guilty to three breaches of planning laws after being taken to court by Westminster council.

The fine is one of the biggest ever imposed for damage to protected historic properties.

But Mr Terry, whose work includes refurbishing the state rooms at 10 Downing Street, insisted it was unfair.

The Regency gate houses, built in 1827, were destroyed last December during modernisation of Hanover Lodge, one of London's grandest homes designed by John Nash. Both were Grade II-listed.

Contractors working for Mr Terry reduced the lodges to piles of rubble with only their back walls left standing after they tried to demolish adjacent modern buildings-The firm, Walter Lilly & Co, was fined £20,000 by Westminster magistrates at the same hearing.

Council cabinet member Robert Davis said: "Westminster has a rich architectural heritage and it is the council's duty to protect this.

"For one of the country's pre-eminent architects to fall foul of the law is disappointing but I hope the size of the fine will send a clear signal to anybody who thinks they can damage or destroy listed buildings, whoever they are."

Mr Terry said the fines were "irritating" because the rule breaches were "purely administrative rather than malicious".

He said: "The problem was the surrounding buildings, from a much later period, needed to be demolished.

"We had permission to do that. But as we were doing the demolition one wall of one of the lodges collapsed. The second lodge was also in a poor condition and had a rotten window. They were both very neglected. My chap said they were dangerous and needed to be shored up with a view to putting them back as they were later.

"The mistake we made was not getting in the necessary 'method statement' to the council to inform them what we were doing. We put our hands up, it was just a straightforward error. The size of the fine was not expected but sometimes the law is an ass."

No one at Walter Lilly & Co was available for comment.

Hanover Lodge, which sits in two acres of grounds, will be worth up to £50million after the modernisation. It has a 150-year lease owned by millionaire Tory peer Lord Bagri. The freehold is owned by the Crown Estate.