Bain Capital Private Equity has proposed buying U.K. personal lines insurer Esure Group PLC for 280 pence per share.

Based on the 419.09 million shares Esure has outstanding, Bain is proposing to offer £1.17 billion. Esure's share price closed up 30.98% at 267.2 pence per share.

Esure said it is in the "advanced stages" of discussing the possible offer. It said Bain's proposed takeover price was a 37% premium on the 204 pence closing price of its shares Aug. 10 — the last business day before the Aug. 13 announcement.

Bain has until 5 p.m. London time Sept. 10 to make a firm offer or state that it does not intend to make one.

The board of Esure has told Bain that it is would be minded to recommend a firm offer if Bain does so at the price and terms set out in the proposal. Esure added that there was no certainty that an offer would be made.