Swedish private equity fund Cevian Capital, which owns 3.4% in Skandia, said it also represented shareholders owning a further 3.5% in Skandia, taking its total backing for an Old Mutual bid to 6.9 percent.
After Swedish newspapers had reported on Wednesday that shareholders representing 13.6% were backing the Old Mutual approach, the Cevian stake would take declared support for the bid to just over 17%.
Earlier in the week, Old Mutual said it had held discussions with Skandia over a potential bid worth around 43 billion Swedish kronor (5.6 billion dollars).
The British group said its proposed offer stood at 42 Swedish kronor per share.
The price would be paid 40% in cash and 60% in Old Mutual shares, it added.