The documentation, which is compiled to adhere to a new US accounting standard known as FIN 48, is believed to have attracted the interest of the US’s Internal Revenue Service and other revenue bodies.

Advisers have warned that HM Revenue & Customs could also start asking for paperwork.

‘Tax authorities obviously know that FIN 48 exists. They are aware that companies now have detailed documentation about what’s in their tax reserve and where their uncertain tax positions are.

‘Obviously any tax inspector would be very interested to get hold of those papers,’ said Jeremy Curd, a senior tax manager at PricewaterhouseCoopers.

In some regions revenue authorities have already demanded and received access to FIN 48 documentation, increasing fears among companies that HMRC and the IRS will soon do the same, or that regions will share details they have accessed.

‘If you look around the world we know of examples of companies that have been asked for tax reserve papers and have given them over. We have to be concerned about how tax authorities share that information and whether it can get back into the UK,’ Curd said.

FIN 48, which became mandatory in December 2006, requires companies that file US GAAP accounts to disclose any uncertain tax positions in their financial statements.

Companies such as Cadbury Schweppes, Vodafone and GlaxoSmithKline, are now required to make detailed disclosures of their tax positions under FIN 48.

The standard has been praised by tax activists, who believe that following the $3.4bn (£1.8bn) settlement GSK made with the IRS and Vodafone’s £2bn dispute with HMRC, it is essential for shareholders to have a clear picture of tax plans.

Although FIN 48 currently only applies to UK companies listed in the US, the IASB is currently considering drafting a similar standard. An HMRC spokesman said the taxman was interested in learning about sources of information used by tax authorities around the world to help identify potential cases of evasion or avoidance.