Australia’s second biggest energy retailer, Origin Energy has received a $13 billion takeover offer from British gas company, BG Group.

In a letter to the ASX this morning Origin said that BG Group, an integrated natural gas company, had approached it yesterday evening with a proposal to buy all of its shares at a cash price of $14.70 each, valuing the target at about $12.91 billion. Origin shares last traded at $10.47 last night.

Not for long: the shares jumped more than 38% to trade at $14.43 (up $3.96) as punters piled into the stock in the hope of a higher offer.

Origin said it has not yet considered the proposal and ”Discussions between the parties will take place and shareholders will be advised of the outcomes. These discussions may or may not lead to an agreed transaction. Origin shareholders should take no action with respect to their Origin shares, pending further announcement.”

In March 2007 Origin walked away from a proposed $15 billion “merger of equals” with AGL.

The BG Group proposal is subject to shareholder and regulatory approvals.

BG recently took up a 9.9% stake in Queensland Gas Co and pumped in hundreds of millions of dollars into QGC’s coal seam methane gas project which is aimed at building an export LNG project, as well as supply gas to business and retailers in southern Queensland. AGL Energy owns around 26% of QGC.

BG also has a minority stake in oil and gas discoveries in very deep water and difficult conditions offshore Brazil which have been described as perhaps the largest discovered in two decades.

It looked at buying the former Hardman Resources in Australia back in 2004.