More than 21,000 HM Revenue and Customs staff could strike over plans to allow private companies to answer taxpayers' enquiries, after a vote by PCS members.

HMRC is planning to trial the use of private contractors in two contact centres from January 2012 to deal with calls about tax credits.

The union says privatisation poses a serious risk to the security of taxpayers' information, and fears it could lead to further privatisation in HMRC and across the public sector.