Doughty Hanson's adviser on its attempt to buy Aegis's Synovate business claims Aegis shareholders were incredulous when they learnt that the £600m approach had been ignored.
Anthony Gahan, the co-founder of Wyvern Partners, said he had contacted Aegis's major institutional investors over the past week to tell them about his efforts to get the communications group into talks on Synovate.
Mr Gahan said: "I think the shareholder response was entirely rational and focused on extracting maximum value from the assets and being surprised that no engagement had taken place with potential bidders. There was a level of incredulity."
He declined to name the investors, but major shareholders in Aegis includes blue-chip names such as Legal & General, Standard Life and Aviva.
He said Wyvern, a boutique corporate adviser, had been sizing up Synovate for 18 months before making an approach on 26 May on behalf of Doughty Hanson, the private equity group.
That approach, to Aegis's chief executive, Jerry Buhlmann, indicated that Wyvern's client was prepared to pay well over £500m for the Synovate market research business, according to Mr Gahan. He said the email went unacknowledged.
Aegis said in early June that it was in exclusive talks with the French company Ipsos to sell Synovate for a price said to be about £520m. Wyvern then wrote to Aegis's board on 23 June and soon afterwards offered to reveal who its client was if Aegis was "willing to engage", but there was no further contact, Mr Gahan said.
He said there appeared to be no effort made to test the market value of Synovate on behalf of shareholders.
Aegis declined to comment yesterday. The company is thought to be unwilling to enter a public spat over Synovate now that it is in exclusive talks with Ipsos. Aegis and Ipsos have stressed that talks over Synovate are at an early stage.
A key player in any decision over Synovate will be Vincent Bolloré, the French investor who owns 26.5 per cent of Aegis. Mr Bolloré could not block the deal, but his blessing will still be needed.
A deal with Ipsos could leave a leaner Aegis open to a buyer. More than two years ago, there was speculation that Aegis might combine with Havas, the French marketing group in which Mr Bolloré is also the biggest investor.
Mr Bolloré has said he has cooled on the idea of a merger between the groups and that his stake in Aegis is financial rather than strategic.
Doughty Hanson's last deal was the £450m buy-out of Vue Entertainment, the cinema group, in November.
Aegis shares fell 3.6p, or 2.8 per cent, to 154.6p. The company yesterday announced a deal to buy MediaVest, a media communications agency, for up to £95m.