Microsoft has been dragged into the growing controversy over internet "blacklisting", with British companies accusing the software giant of wrongly barring them from communicating with users of its Hotmail e-mail accounts.

Just a day after Google came under the spotlight for dropping BMW in Germany from its search engine results, entirely legitimate British companies are complaining that Microsoft is treating them like "spam" e-mailers - in a move that could cost them collectively millions of pounds in lost sales.

One marketing director at a leading online retailer described the measures being taken by Microsoft, and others, as "Draconian" and said he was unable to communicate with almost 1 million of his customers who had already bought items from his company.

As of December last year, there were 8.8 million Hotmail accounts in the UK alone.

The businesses, which range from consultancies to online retailers, say Microsoft is unfairly blocking them from sending e-mails to customers with Hotmail addresses as part of a flawed attempt to filter out unwanted spam messages. They say this is happening even when a customer has specifically agreed to receive mails.

Businesses accuse Microsoft of refusing to explain its criteria for the Hotmail blacklisting, which prevents them from sending out operational as well as marketing e-mails to individual customers or business subscribers.

And they complain that, as well as the possible loss of trade and trust with their customers, blacklisted businesses have to pay a fee running into thousands to then gain a so-called "whitelisting".

Chris Lake, who runs e-consultancy, which advises companies on their internet strategies, said: "About 10 per cent of our users have Hotmail addresses. We send regular marketing e-mail and not-so-regular operational e-mail. Both have been stopped by Hotmail. We need operational e-mail in particular to get through, such as, 'your card has expired so we cannot renew your subscription - please update your card details'. People who don't get this become lapsed subscribers.

"We asked Hotmail why we'd been blacklisted. It turns out that we had generated just two 'complaints' out of almost 1 million emails sent. Team Hotmail will not tell us what consitutes a complaint."

Mr Lake said his company had had to pay nearly £1,000 to go through the process of becoming a "bonded sender", which certifies the validity of the company sending out the e-mails. He said that, as well as taking about 90 days, the process involves "various hoops".

"If this is troublesome for e-consultancy, God knows what it's going to be like for the tens of thousands of small and medium-sized entreprises out there that will have been blacklisted. Much less the big retailers.

"What's the potential cost to businesses if mail doesn't get through? Keep in mind that there is financial damage and brand damage ... and it is not necessarily the fault of the e-mailer."

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