Investment firm Cathexis launched the £71m offer just before Christmas.
ISG wrote to its shareholders on December 23 urging them to reject the offer.
The defence circular stated that the Cathexis offer undervalued the company based on analysts’ profit forecasts for 2016 and 2017.
Under the Takeover Code “reference by an offeree company to an investment analyst forecast constitutes a profit forecast by the company itself” and must be backed-up by ISG’s accountants and director.
ISG said this morning that it “is not able to comply with these requirements.”
The defence circular has now been amended and a revised document will be sent to shareholders today.
ISG added: “Shareholders should rely only on the Revised Circular in reaching any conclusions on the merits of the Offer (and, in particular, shareholders should not place any reliance on the consensus forecasts for ISG for 2016 and 2017 set out in section 3.1 of the Original Circular since ISG has not been able to substantiate these forecasts).
“In all other respects the Board, which has been advised throughout by Numis, stands by its rejection of the Offer”