Nearly a third of UK businesses have failed to make the most basic preparations for this summer's London Olympics despite the government hoping to generate an extra billion pounds in trade and investment, a survey said on Tuesday.

The government, which has spent 9.3 billion pounds on the Olympics, is to host a series of trade and investment meetings in an attempt to help business cash in and generate long-term growth during an economic downturn.

But while 73 percent of the 1,200 businesses questioned in an online BT survey said they expected to prosper as a result of London 2012, 29 percent were still unprepared.

Success or failure depends on how ready they are, Jeff Kelly, chief executive BT global services, said. There's still time to get plans in place.

Visa, an Olympic sponsor, predicted sports fans will spend an extra 750 million pounds during the Olympics.

About 93 percent of businesses expected negative knock-on effects during the Games, including supply chain disruption, higher hotel costs, staff attendance and productivity.

About 40 percent predicted staff will want time off or will be unable to get to work because of congestion on London's creaking transport system, but nearly 30 percent have no plans at all to deal with this.

Businesses also seem to be ignoring the potential threat of a cyber attack despite the government putting such a danger at the top of its list of threats to Britain's security.

BT, an Olympic sponsor, said confidence in IT resilience may be misplaced because only 34 percent of organisations had assessed the impact of security risks such as a cyber attack.

British businesses seemed to be pessimistic about the long-term benefits of the Olympics, with only 28 percent saying they believed they would continue to enjoy benefits a year or more after the athletes and spectators have gone home.

The BT survey also looked back at the Vancouver 2010 Winter Games where 61 percent of businesses felt they benefited longer-term.

Some British businesses might be relying on athletes taking the strain. About 65 percent of businesses said the Games could be a catalyst for positive change in the economy if Team GB does well.

(Reporting by Avril Ormsby)