UFJ is proving a desirable asset

The 3.2 trillion-yen ($29bn; �16bn) bid comes after Mitsubishi Tokyo Financial Group (MTFG) earlier this month agreed to take over troubled UFJ.

Sumitomo has offered one of its shares for every UFJ equity; a premium of 23% based on UFJ's Tuesday closing price.

The move had been widely expected, with Sumitomo wooing UFJ shareholders.

"It seems like a pretty aggressive bid," said Barclays Capital analyst Jason Rogers.

"This is more or less a takeover bid as opposed to a merger."

Legal challenge

The takeover battle comes as Japan's top banks seek to improve their financial strength and clear up their bad debt burdens.

"The merger of our group and UFJ would be the best choice for the shareholders, customers, and employees of both companies," Sumitomo Mitsui said.

"We strongly hope our proposal will be positively considered."

UFJ said it would "carefully consider" the offer but that it had not changed its plans to merge with MTFG.

But the merger between MTFG and UFJ is facing legal action from Sumitomo Trust & Banking - a division of Sumitomo Mitsui.

Sumitomo Trust wants to overturn a ruling which allows UFJ to include its trust bank unit in the merger with MTFG.

UFJ had originally agreed to sell its trust bank to Sumitomo Trust, but scrapped the deal when it agreed the merger with MTFG.