Industrial action by postal workers has cost London's businesses more than £500 million since the start of the summer, it has been claimed.
Last week's strikes alone cost firms in the capital an estimated £200 million, according to the London Chamber of Commerce and Industry.
The body, which represents around 3,000 businesses, also called on Business Secretary Lord Mandelson to step in and help resolve the dispute.
London has been hit by series of localised strikes as part of ongoing disagreements between the Royal Mail and the Communication Workers Union (CWU) over modernisation plans.
Postal workers also staged two days of national strikes last week, with talks planned for Tuesday in a bid to avert a further three days of industrial action at the end of this week.
The LCCI said firms were having to pay more for alternative ways to deliver mail, and were also suffering from cash-flow and other problems.
Chief executive Colin Stanbridge said: "This is a colossal amount of money for the London economy to lose and will delay the capital's economic recovery.
"Not being able to rely on a normal postal service forces companies to pay extra for couriers, delays consumer spending, damages client relationships and plays havoc with a firm's cash flow.
"It is now high time that Lord Mandelson intervened personally to broker a deal between Royal Mail and the CWU and bring an immediate end to these strikes."
The figures came from a survey of London businesses by pollsters ComRes conducted for the LCCI.