Prime Minister Tony Blair's TV appearance at Microsoft's Reading HQ has been criticised by opposition parties as another example of the "unhealthy" relationship between Microsoft and the UK Government.

Blair sat through a Microsoft Office XP demonstration yesterday, following the earlier launch of the Labour Party's business manifesto, and said that the technology could have a "huge impact" in a range of applications.

Aides moved quickly to explain that the Prime Minister was not endorsing the product, but critics said he appeared to have crossed the line from taking an interest in the software to appearing to back it.

David Heathcoat-Amory, Conservative Spokesman for Trade and Industry, said: "In launching their stale and recycled business manifesto, I am surprised that Labour chose to use it to promote a commercial product. It is further evidence of the Labour Party's unhealthy relationship with big business."

Charles Kennedy, leader of the Liberal Democrats, said: "Any politician has to be careful about associating themselves with such a dominant company as Microsoft."

But the Blair government has been nothing if not keen to associate itself with the Redmond giant. Indeed, when Compaq pulled out of its flagship secure transaction Government Gateway portal, it was to Microsoft that Downing Street turned.

The Gateway project is a centralised registration service for all e-government services in the UK, and is a key part of Labour's ambition to put 100 per cent of its services online by 2005. E-envoy Andrew Pinder attended an international conference in March to pitch the system to other governments in conjunction with Microsoft.

The Gateway currently relies on digital certificates from either Equifax or ChamberSign to authenticate transactions, but Equifax certificates can currently only be used with Internet Explorer 5.01 or later, and ChamberSign certificates are not currently supported on version 6 of the Netscape Navigator browser.

An investigation by LinuxUser magazine has slammed the Gateway for imposing a "Microsoft tax" on potential users.

The article said: "As we go to press, the authentication service www.gateway.gov.uk, flagship of the UK Government's ambitious policy of providing a complete range of services electronically by 2005, restricts access to anybody not running a combination of Windows and Internet Explorer.

"Mac users can enter the Gateway site but cannot do anything useful. Windows users running any version of Mozilla or Opera are barred from entry. Anybody else, including those running Linux or Unix of any flavour, is not welcome. They are told that they are running an unsupported browser.

"Critics say that such a state of affairs is untenable, not just because companies that use other operating systems cannot gain access to pay their tax and the like, but also because the implication is that the Government is committed to one vendor."