Insurers' exposure to the massive blackout across North America last week is still unknown as the industry waits for investigators to identify the cause.

But the power cuts will spark business interruption claims worth millions of dollars, according to sources.

And if power companies are found to be negligent, costs to insurers could be much higher.

Both industry and service sectors on the US north-east coast and in Canada were affected.

General Motors closed 17 of its North American plants, British Airways was forced to cancel nine transatlantic flights and many shops and restaurants had to dump food stocks.