Hellmann & Friedman, the US private equity firm, has emerged as a potential backer to one of three trade buyers in the final stages of bidding for NOP World, the market research business being sold by United Business Media.

Banking sources said the buy-out firm was most likely backing Ipsos, the French market research company which is thought to be advised by UBS, although sources close to Hellman & Friedman said the firm had been approached by more than one company.