Polygon has increased its interest in the company to just more than 1.5 per cent of the shares.
The supermarket's stock has rocketed since CVC Capital Partners, Kohlberg Kravis Roberts and Blackstone announced that they were considering a takeover bid earlier this month, estimated to be in the region of£10 billion.
Polygon is not the first investor to take a big bet on a takeover approach for Sainsbury's, but its wager is larger than the 1 per cent interest of Qatari investment company Delta Two.
Many retail analysts believe that a bid for Sainsbury's is increasingly likely after two weeks of intense speculation and analysis.
Oriel Securities analyst Jonathan Pritchard said: 'I think that a bid will be forthcoming - these guys [CVC, KKR and Blackstone] are serious.'