The head of one of the UK's largest retail chains has said the congestion charge is harming his business.
Sir Stuart warned against extending the scheme too soon
Sales at the flagship John Lewis store on Oxford Street have dropped 9% below those of its other stores in the first six months of the scheme.
The £5-a-day charge to drive into central London was introduced on 17 February.
Sir Stuart Hampson, chairman of the John Lewis Partnership, told BBC London: "What we've said is if you took John Lewis's sales and compare them to the group over the whole country, we are running about 9% below what we would expect.
"There's been a heatwave everywhere, there's been the general economic effects, so I wouldn't say the whole of that difference is congestion charge but there's definitely a congestion charge effect.
"And it's not only us, other retailers are saying as a result of the congestion charge there have been lower sales in central London."
The scheme has cut journey times by a third in central London.
Threat to investment
While various reports suggest many companies support the charge, others - usually small businesses - have reported a drop in custom.
But Arriva said the charge had increased demands for its buses and helped to lift the company's profits.
Sir Stuart said he was not opposed to the charge but called for any extension of the scheme to be delayed until the full impact could be measured.
The store has commissioned its own research into the effects of the charge.
Sir Stuart also told BBC London not all shoppers realised the congestion charge did not apply at weekends and called on London Mayor Ken Livingstone to start an advertising campaign saying so.
London Chamber of Commerce chief executive Colin Stanbridge said: "Today's announcement by John Lewis is the most concrete proof so far that even the most famous names are susceptible to the downturn in trade to which the congestion charge has contributed."