Microsoft is planning to make its controversial Software Assurance programme compulsory for business that want to buy the enterprise version of Windows Vista.

Rather than run the programme as an alternative method of purchase by companies that wish to keep up-to-date with Microsoft software upgrades, the software giant today announced the opposite would be true. "Windows Vista Enterprise is available exclusively as a benefit of Software Assurance," read a statement.

Another part drilled the message home: "Windows Vista Enterprise is a new edition of Windows, available exclusively to Software Assurance customers."

At the same time, Microsoft outlined an expanded set of services that would be added to Software Assurance, including technical support, training, deployment planning - the news of which leaked earlier this week.

Nonetheless, the announcement is certain to unsettle some customers. The Software Assurance programme, first introduced in 2001, encourages customers to pay an annual fee to use Microsoft's software rather than buying outright licenses. The annual fee includes software upgrades plus other support services.

But it has proved controversial one and met with considerable resistance after some analysts said customers would end up paying more under Software Assurance than under the previous licensing plan. Many businesses are angry that they have not received as much as they had expected from the programme thanks to delays in the rollout of new Microsoft products. Such problems led House of Fraser to announce just last week that it had dropped Software Assurance in favour of paying the full rate for upgrades.

Now Microsoft is not just encouraging customers to use the programme but requiring them to. "If anyone wants to move onto the next generation software, then they've got no alternative," said Michael Azoff, senior research analyst with Butler. "It's sort of a clever move by Microsoft."

Whether business customers are willing to go along with the change is another matter.