A spokeswoman for the mobile telecoms giant said the investigation was "ongoing", but declined to comment on who was the subject of the enquiry. "The matter is with our legal department," she said.

Last month Vodafone was reported to be implementing a new print buying system developed by The Print Specialists.

It has since come to light that Mark Coulson, Vodafone's head of marketing procurement, and buyer Martin Simper were directors of The Print Specialists.

But it is not known if the investigation is linked to Coulson and Simper's involvement in the firm.

Companies House records show that Coulson and Simper were appointed as directors of The Print Specialists on 28 September this year, when the business was incorporated.

The firm's original registered office was also the same as Coulson's home address in Thatcham, until 16 November.

Coulson and Simper resigned their directorships of The Print Specialists on 12 November and 18 October respectively.

A colleague is now carrying out Coulson's duties at Vodafone. He said Coulson was out of the office and it was unclear when he would return.

An audit was conducted earlier this year into Vodafone's print procurement by consultant Rob Sirett's firm I.See Direct.

Sirett subsequently acquired The Print Specialists, taking up his position as director on 18 October according to company records. He declined to comment on the Vodafone investigation.

Story by Andy Scott