The UK government does not know how much open-source software it uses or what its value is, ministers have admitted.

Gillian Merron, minister for transformational government in the Cabinet Office, said there were no targets to promote the use of open source software such as the Linux operating system or the OpenOffice productivity suite, "except where it is the most cost-effective way" of meeting government needs.

In answer to questions from Liberal Democrat MP Julia Goldsworthy, Merron said: "Information on the total use and value of open source in government is not held centrally and could not be obtained except at disproportionate cost."

But open source software did "play an important role" in major government applications including the Directgov web portal, the online vehicle licensing system and Jobcentre Plus's system to help people to find work, Merron added.

The minister's answers come three years after the government pledged to avoid "lock-in" to proprietary systems. In 2004, the government published a policy document promising to consider open source alongside proprietary products in IT procurements. Contracts would be awarded "on a value for money basis," the document said, adding: "UK Government will seek to avoid lock-in to proprietary IT products and services."

Research produced by the Lib Dems shows most government departments use a minimal amount of open source software, with the Department of Work and Pensions using none at all and figures of less than 1 percent at the former Department for Education and Skills and Northern Ireland Office.

Merron said: "Government seeks the most cost-effective IT solutions. It tries to avoid mandating particular technologies so that suppliers can manage technology risks."

Asked what assessment the Cabinet Office had made of the potential of open source to produce efficiency savings, Merron replied that Open Source could be cheaper to buy but could also "entail higher support and other costs."

She added: "The additional costs or savings depend on the individual business requirement. This is supported by external studies which have not shown a consistent advantage to open source in these terms."

Over the past three years, the government had "continued to explore" the use of open source to reduce total costs and improve the quality of business systems, Merron said.

She restated the government's policy to consider open source alongside proprietary products in procurements on a value for money basis, to use interoperability products based on open standards and specifications, and to seek to avoid lock-in to proprietary IT products and services. The government would also consider buying the full rights to bespoke software code or customisation of commercial off the shelf software it had procured "wherever this achieves best value for money," the minister said.