TOKYO, Dec 13 (Reuters) - Business confidence at big Japanese manufacturers improved in the three months to December for a third straight quarter, the Bank of Japan's closely watched tankan quarterly business survey showed.

The headline index for big manufacturers' sentiment was plus 12 in the December quarter, the tankan survey showed on Wednesday. The index was seen deteriorating, likely underscoring concerns about the risk to outlook such as slowdown in the Chinese economy.

The survey also showed big firms plan to raise their capital spending by 13.5% in the financial year to March 2024. That compared with the market's median forecast for a 12.4% gain. (Reporting by Leika Kihara and Tetsushi Kajimoto; editing by Jonathan Oatis)