The UK financial watchdog is set to look into trading in Somerfield's shares made ahead of the news of a takeover approach, reports have said.
Somerfield announced on Tuesday that it had received a bid approach last Friday.
The Financial Services Authority (FSA) is expected to look into high trading volumes on Monday, and the purchase of 100,000 shares in the company on Thursday by its executive chairman John von Spreckelson.
Somerfield has defended Mr von Spreckelson, insisting the trade was "pure coincidence".
Shares in Somerfield jumped more than 20% on Tuesday following the company's statement.
But speculation about a possible bid had triggered 25 million Somerfield shares to be traded the day before.
Somerfield's share price had been rising steadily in the days running up to Tuesday.
Its shares rose nearly 12% from the start of trade last Wednesday to the close of trade on Monday, while the FTSE 250 index, of which Somerfield is a member, rose just 1%.
Meanwhile, speculation as to the identity of Somerfield's suitor has centred on retail executive John Lovering.
Mr Lovering's consortium is reported to value the Somerfield group - which includes the Kwik Save chain - at about £500m.
Somerfield is the UK's sixth largest supermarket group with about 1,400 stores and 56,000 employees.