Ophir Energy confirmed that it had received a takeover approach from Indonesia's Medco valuing it at around £343m.
Ophir said Medco had made an unsolicited approach back in October at 58p per Ophir share but had reduced it to 53.8p in December following a fall in the oil price.
Medco had now submitted an updated 'possible offer' at 48.5p per share but had not yet made a firm bid, Ophir said.
Ophir had last year granted Medco access to due diligence.
On Friday, it said Medco had made a unilateral announcement about its latest offer, without any further discussion.
At 3:03pm: (LON:OPHR) Ophir Energy share price was +0.33p at 43.33p