Santander, the euro zone's biggest bank by market value, launched the sale on June 30 and the tight deadline is a sign the lender wants to quickly draw a line under potential risks linked to the takeover of Banco Popular on June 7.

It had initially set itself a 3 year deadline to sell all of Popular's bad property assets, one of the factors that brought down the 90-year-old lender.

The sources said private equity funds Blackstone, LoneStar and Apollo were invited to submit binding bids for a 51 percent stake in the portfolio, which includes repossessed assets worth 18 billion euros and 12 billion euros in non-performing loans.

With a coverage ratio for losses of 69 percent and a net value of 9.2 billion euros, offers are expected in the ball park of 5 billion euros, although the final price will not be set until a comprehensive due diligence has been completed, the sources said.

Banco Santander, Blackstone, LoneStar and Apollo declined to comment.

(Additional reporting By Pamela Barbaglia and Steve Slater in London; Editing by Julien Toyer)

By Anjuli Davies and Jesús Aguado