Two-thirds of Microsoft's largest customers have yet to sign up for the company's controversial Software Assurance programme with only two months to go before the summer deadline.

The figure was uncovered by reports from two industry watchers, Gartner and Giga.

One third of business customers said that they did not intend to sign up for the subscription programme and another third remain undecided, according to the findings.

Worryingly for Microsoft, Giga described the opponents to the scheme as "no small number".

It is now a year since Microsoft first launched its Software Assurance plan which asked companies to commit to buying upgrades ahead of schedule in return for an annual fee. Those that do not join will have to pay the full amount for any upgrades.

Experts said that the plan could drive IT managers to consider rival packages such as Sun Microsystems' StarOffice, and to switch operating systems to the open source alternative, Linux.

Microsoft defended the programme, however. "Moving customers from one-time upgrades to an annuity model is critical for us," said Rebecca LaBrunerie, product manager for worldwide licensing and pricing.