Accor said it would fight any take over bid after two investment companies increased their stake in the hotel chain to 17.52%.
Eurazeo and Colony Capital, which have signed a five year shareholders' agreement, said they planned eventually a 30% holding in the French hotel company.
Colony said it planned to increase its stake to 20% in the coming months but denied there were any plans to take over the chain.
Accor directors said that while the increased stake confirmed the company's "favourable growth potential", they were "committed to preventing a takeover of Accor that would avoid having to make a public offer for all of its outstanding shares."
Accor is a major global hotel chain with brands which include Sofitel, Novotel, Mercure, Ibis and the recently launched Pullman.
It has more than 4,000 properties with 500,000 rooms in 90 countries.
Accor has a market capital of about 13bn and has enjoyed several years of steady profit. For the first quarter of 2008, it reported a 4.8% increase in revenues from 1,791m to 1887m from its hotels and services companies.
Eurazeo, a Paris-based investment company, has been investing in Accor through two other investment companies Colyzeo and ColTime since June 2005.
It now owns 19.6m shares, an 8.5% stake. 9.6m have been bought on the market while 10m have been brought through a share purchase programme which allows Eurazeo to buy a further 1m shares.
Its stake in Accor is worth 1,077m.
Eurazeo has approximately 6 billion in assets and is a major or majority shareholder in companies including APCOA, B&B Hotels, ELIS, Europcar, Rexel, ANF, Air Liquide and Danone.
It plans to increase its stake in Accor to 10% and has requested board representation.
Colony, based in Los Angeles and with offices in major cities across America and Europe, including Paris, has also been investing in Accor since 2006 through ColTime.
It now has 20,634,000 shares, 9% of the company as well as two board representatives.
Together their stake gives Eurazeo and Colony 15.71% voting rights.