WPP faces a claim for damages from the IR services firm - bought for £2.7m - over the share element of the deal.
IP chairman Pia Marocco and trust company Butterfield sold a 49 per cent stake in the firm to WPP nearly four years ago.
Marocco and Butterfield allege, in a High Court writ, that they are entitled to further shares in WPP on top of those already received.
While the cash side of the deal has already been settled, the case centres around the issue of the qualifying period, in which shares can be claimed.
In the writ, Marocco and Butterfield claim that they should have received their shares last September.
WPP argue that the shares should have been issued in July.
It is understood the difference determines how many shares Marocco and Butterfield are entitled to receive.
WPP and IP both refused to comment.
IP provides investor relations marketing services, specialising in roadshows aimed at the private and public sector.
The agency in particular organises investor roadshows to help promote privatisations and stock market flotations.
Meanwhile, WPP boss Sir Martin Sorrell this week sent a warning to the communications industry that it needs to 'look at the bigger picture' rather than focusing on individual disciplines, such as PR and advertising.
In an article in The Financial Times, Sorrell said clients and consultancies need to think in terms of the end result and combine marcoms skills to achieve that.