Small firms are "seriously underestimating" how much a major incident like a flood or fire would cost their business.
Telecoms giant BT said its study of 1,400 companies found that one in three believed they would not be hit by any extra costs if they were hit by a disaster.
One in ten estimated that a disaster might cost them up to £5,000, but the official estimate is between £8,000 and £17,000.
Bill Murphy, managing director of BT Business, said: “It’s an issue that many small companies just don’t address until disaster strikes, but by then it's too late.
Simon Briault, of the Federation of Small Businesses, said: “After the terrible floods of last summer, we believe small business owners are more aware than ever of the dangers of natural disasters or serious incidents.
“The insurance industry should do all it can to make contingency planning for small businesses easier and more affordable.”
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