Sept 8 (Reuters) - BGC Partners Inc is likely to make a $675 million all-cash unsolicited offer for derivatives broker and rival GFI Group Inc in a bid to bolster its business, the Wall Street Journal reported on Monday citing people familiar with the matter.

BGC's offer of $5.25 a share could throw into doubt GFI's recent agreement to sell itself to Chicago exchange operator CME Group Inc, the WSJ said. (http://on.wsj.com/1rxdxtu)

BGC planned to make the proposal late Monday in a letter to GFI's board and would go directly to GFI shareholders if its offer was rejected, the financial daily said quoting unidentified sources.

CME Group, the world's largest futures exchange operator, said in July it would buy GFI Group to expand its reach in the European energy and global foreign exchange markets.

The owner of the Chicago Board of Trade and Chicago Mercantile Exchange had said it would acquire GFI in an all-stock deal worth $580 million and assume $240 million in outstanding debt, for a total value of around $820 million.

GFI, BGC and CME were not immediately available for comment beyond regular business hours.

New York-based BGC, which was spun off from Cantor Fitzgerald LP a decade ago, currently has over $640 million of cash on hand, WSJ said.

The buyout effort would be the latest from BGC. Its previous proposals were turned down by GFI, the newspaper said.

GFI's shares had closed up 10.5 percent at $5.03 on the New York Stock Exchange on Monday.

(Reporting by Shailaja Sharma in Bangalore)