By Pamela Barbaglia and Jesús Aguado

Caser is one of Spain's leading players in bancassurance providing anything from life, health and car insurance policies and relying on a network of regional banks to sell its products.

Switzerland's Helvetia and Spain's Santa Lucia are also vying for control of the 77-year-old company which is backed by French insurer Covea and a series of Spanish banks.

The auction, led by Nomura and Barclays, is expected to wrap up before Christmas valuing the entire business at up to 1.2 billion euros (1.03 billion pounds), the sources said.

Madrid-based Caser is looking for a long-term investor that could buy about 60% of the company, one of the sources said.

Private equity investors including London-based Cinven have not been invited to submit bids as their investment horizon typically spans three to five years, he said.

Caser was not available for comment while Ageas, NN Group, Helvetia, Santa Lucia and Cinven declined to comment.

Caser has a fragmented shareholder base with French insurer Covea ranking as its single biggest investor with a 20% stake.

Covea wants to cash out and has teamed up with three Spanish banks - Bankia, Caixabank and Abanca - that hold a combined 36.4% of Caser to find a new owner.

Three other regional banks - Liberbank , Unicaja and Ibercaja - control an overall 36.1% stake of Caser and have close business ties with the insurer as they distribute its products across their client network.

These banks are expected to retain some exposure to the insurer as a way to protect their own interest, the sources said.

"Liberbank, Unicaja and Ibercaja need to have skin in the game as they play a major role as Caser's distributors," one source said.

He added Liberbank, Unicaja and Ibercaja have agreed to trim their interests to less than 10% each.

Banco Sabadell and BBVA have a stake of 1.8% and 0.2%, respectively, and may also sell depending on the final valuation, another source said.

Caser employs about 1,600 people and had revenues of 1.83 billion euros in the last fiscal year.

Bankia, Caixabank, Abanca, Liberbank, Unicaja, Ibercaja, Sabadell and BBVA declined to comment while Covea was not immediately available.

By Pamela Barbaglia and Jesús Aguado