A third of City of London firms have made no contingency plans for the Olympics, and only one in 10 are planning to allow staff to work from home.
Research by the MWB Business Exchange of 430 London companies in potential travel hotspots finds that they are under-prepared for the Games in ways that could cost the economy billions of pounds.
A London-wide Olympic stress test of businesses in readiness for the Games starts today – 80 days before the opening ceremony. MWB says this is too little, too late.
Olympic organisers have been urging employers, particularly those in Canary Wharf, to allow staff to be flexible.
MWB's head of Olympics, Kathryn Hurt, said: "We've been calling on companies to stress-test their systems for months. Now with just 80 days to go, businesses are slowly waking up to the fact that they need to prepare. But it could be too little, too late.
"Our survey shows that many London businesses are sleepwalking into a potential work nightmare. Businesses who have not thought about it are really going to be hit the hardest."
She added: "During snowday in February 2009, £1bn was lost from the economy. Now times that by the 17 days of the Olympics, and you can see the potential hit UK plc could take."
* Only 11 per cent of firms are allowing their staff to work from home;
* 30 per cent have not made any contingency plans for the Olympics even though 89 per cent believe the games will affect them;
* Less than a third (30 per cent) are allowing employees to work flexible hours to beat travel disruption;
* Just 18 per cent of firms have reviewed their staff's annual leave arrangements.
There will be an extra 20 million train and Tube trips during the course of the Olympics, and 80 per cent of spectators visiting the Games will travel by train and Underground.