Executives at KKR and Blackstone sent emails that allegedly show they colluded in a bid to keep prices low in the mid-noughties buyout boom, according to court documents.

The emails have emerged as part of a class action lawsuit filed by shareholders in companies bought by private equity firms. 

In documents that were unsealed yesterday, it was revealed that Blackstone president Tony James wrote to KKR co-founder George Roberts: “We would much rather work with you guys than against you. Together we can be unstoppable but in opposition we can cost each other a lot of money."

The email was allegedly sent in the run-up to the $17.6bn (€13.7bn) acquisition of semiconductor company Freescale, a sales process that Blackstone won after KKR stepped down. 

The claim has been brought by 24 companies that were bought by private equity firms between 2003 and 2007 who claim that they lost at least $1bn because of collusion by more than ten firms including KKR, TPG Capital, Bain Capital and Carlyle Group.

James Coulter, co-founder of TPG, wrote in an email that an aggressive bid for SunGard, which was acquired by seven firms for $11.4bn, would make enemies "while perhaps benefiting no one but the (company's) shareholders", the documents reveal. 

Meanwhile, in the $21.3bn buyout of healthcare group HCA, James Attwood, a managing director at Carlyle Group, wrote to Alex Navab, co-head of KKR's North American private equity business: "We will not in any way interfere with your deal. We would, of course, love to join you if you need more equity, but rest assured that you will not see us in any other context on HCA."

Click here to read our analysis of collusion in club deals.