TOKYO, June 17 (Reuters) - The Bank of Japan is likely to trim its monthly bond buying by around 2 trillion yen ($12.7 billion) in new guidance due next month, but forgo raising interest rates at least until September, former board member Makoto Sakurai said on Monday.

At its policy meeting on Friday, the BOJ decided to start trimming its huge bond purchases and announce a detailed plan in July on reducing its nearly $5 trillion balance sheet, taking another step toward unwinding its massive monetary stimulus.

Governor Kazuo Ueda gave few clues on how much the BOJ will actually trim its bond buying, saying only that the taper size will be significant.

"The BOJ has the option of reducing its monthly purchase amount by just one trillion yen. But with the governor having said the size would be 'significant,' there's a good chance it will taper by around 2 trillion yen," Sakurai told Reuters in an interview.

The BOJ currently buys roughly 6 trillion yen of government bonds per month with an allowance of 5-7 trillion yen.

The BOJ's decision to announce its bond-tapering plan at its next meeting in July 30-31 has heightened uncertainty on whether it will hike short-term interest rates at the same meeting, or hold off until later in the year to avoid upending markets.

Sakurai, who retains close ties with incumbent policymakers, said the BOJ will likely forgo raising rates in July and wait for more clarity on whether summer bonus payments and wage gains will help consumption will rebound.

"The next interest rate hike will likely happen in autumn or early next year," he said.

After ending eight years of negative interest rates in March, the BOJ currently sets the short-term policy rate target in a range of 0-0.1%.

($1 = 157.4300 yen) (Reporting by Leika Kihara and Takahiko Wada; Editing by Stephen Coates)