A quarter of website crashes and overloads are caused by bad internal communications, according to a study by web testing service SciVisum.
Some 73 per cent of companies had experienced website failures during online campaigns, and 26 per cent of marketing departments confessed that they never alert the IT department about impending campaigns.
A further 52 per cent only 'sometimes' or 'rarely' work with the IT department before the start of a new campaign, and only 22 per cent of those surveyed always involve the IT department.
"Forget hackers, British firms are shooting themselves in the foot when it comes to keeping their websites running," said Deri Jones, chief executive at SciVisum.
"It's counterproductive to try and attract customers to your site that you can't support and end up driving away.
"UK businesses urgently need to bridge this yawning gap between their marketing and IT departments or they will end up destroying customer confidence and damaging their corporate reputations."
Nearly two thirds of marketing personnel said they have no idea how many user transactions their website can support, despite an average transaction value of £50 to £100.
The study found that 47 per cent do not know how many users abandon their sites with incomplete transactions. Some 18 per cent estimated that at least half their users were abandoning transactions, and a further 16 per cent estimated up to a quarter of users.
Michael Allen, global performance director at Compuware, said: "This is really about risk management. Businesses should be using risk prediction and testing technologies to work out how many users their websites can cope with. It's all about giving the marketing people visibility of the risks of failure."
Although 49 per cent had looked into why users were abandoning transactions, almost half were unable to discover the cause from their IT departments, being informed that it was too complicated or that the investigation was inconclusive.
Spencer Gallagher, managing director of web firm Bluehalo, said: "It's really about getting these different groups to talk to each other. The IT teams often come in late on a project and are more focused on some of the technical issues, so there can be communications problems."
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago
Such an earthquake would lead to a complete stress release in this segment of the fault system
Four types of test were performed to assess the performance of parachutes that could be used in missions to Mars
Warming was most pronounced in Siberia region