Research shows that almost two-thirds of businesses in London expect virtually no impact on their ability to operate during the Games, which will last for six weeks.

Only a quarter of firms have appointed a person or group to manage the potential risks to their business and assess the opportunities on offer.

Rick Cudworth, Deloitte's Head of Business Continuity and Resilience, said that bosses needed a wake-up call.

"They operate in service industries where people are vital, where the supply chain is time critical and where having products on the shelf or food to serve in restaurants is essential to their daily business," he said.

"Thinking through the impacts that an Olympic-scale event could have on logistics, the supply of goods and the movement of staff is essential.

"We believe that - with insight and planning - businesses can maximise the opportunities to benefit commercially from London 2012.

"The clock is ticking and the planning needs to start now."

Seventy per cent of retailers across the UK expect no impact despite the industry facing direct challenges such as getting high numbers of staff into stores and product on the shelves.

Transport was the biggest worry for companies but very few were concerned about supply disruption or resource scarcity - such as hotel rooms - which will definitely be in short supply.

Over half of companies surveyed expect the Games to deliver an increase in demand for their services – but even here there is a disconnect with many companies expecting an increase in demand, but not planning how they are going to secure that extra business.

Deloitte thinks many companies are overlooking the potential risks to their business operations and aren't planning well enough to take advantage of the opportunities – both of which could have repercussions for them in 2012.

Heather Hancock, London 2012 Partner at Deloitte, said: "The London 2012 Games will be a great sporting, spectator and viewer event - it will set the capital and the whole country alight.

"It's a good start that, with just under two years to go, 25 per cent of the businesses we surveyed are already thinking about the impact and the opportunities of London 2012.

"These companies stand to be best placed to benefit from the business boost the Games can deliver.

"However, an event of this scale, with sites across London and throughout the country, will present some serious challenges to those companies who operate in and around venues for the Games if they fail to understand and plan ahead.

"This makes some of our findings really surprising."