Bidders are likely to include the likes of Taylor Nelson Sofres, Aegis, WPP Group from the UK, French firm Ipsos and German group GfK.
Analysts' forecast the firm could attract anything between £250m to £350m.
NOP World would be a prize asset for any group. It is one of the world's top 10 market research firms, with brands including NOP Research Group, Strategic Marketing Corporation, Mediamark Research Inc, Roper/ASW, Market Measures/Cozint and Allison Fisher.
Last month, United announced it had appointed financial advisers Dresdner Kleinwort Wasserstein and Allen & Company to conduct a strategic review of NOP, fuelling rumours that chief executive Lord Clive Hollick was set to sell off the market research firm before he stands down on April 5.
New United chief executive Davin Levin is said to be less enthusiastic about market research than Lord Hollick, who helped build NOP into one of the world's top firms.
NOP is best known in the UK for its political polling work and it has long partnered with the BBC and the Labour Party. It recently, along with rival MORI, signed a deal with the BBC and ITV News to do a joint exit poll for the next general election to minimise the risk of calling the wrong result.
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