UPM-Kymmenes general counsel, Reko Aalto-Setl, said it could be months before anything further was heard from the US authorities.

At the moment its just questions, but the indications from our US lawyers is that it could be a lengthy process, he added.

Aalto-Setl said the company would respond to the subpoena in due course.

No estimates have been put on how much this will cost UPM-Kymmene. The Finnish paper manufacturer, which also owns Raflatac, had its proposed acquisition of US label substrate producer MACtac stopped after a US District Court approved an injunction to block the 260m ($US420m) deal in July.

MACtac is owned by Bemis Company, another labelstock producer that has been under investigation by the authorities.

Adhesive labels producer Avery Dennison said it would co-operate fully with the investigation.