TOKYO, Nov 18 (IFR) - Japanese government bond prices slipped on Monday, with the 20-year yield underperforming ahead of a 20-year debt auction the following day.

The 10-year yield was up 0.5 basis point at 0.635 percent after touching 0.640 percent earlier, while 10-year JGB futures were largely steady at 144.68.

The 20-year yield added 1.5 basis points to 1.515 percent, its highest since Oct. 16 and ahead of Tuesday's 1.2 trillion yen ($11.97 billion) bond sale of similar maturities.

Also, a lack of purchases by the Bank of Japan in the longer-dated zone in its bond-buying programme to revitalise Japan's stuttering growth on Monday, also led to higher yields in the long-maturities.

The central bank offered to buy 400 billion yen of JGBs with residual maturities of five to 10 years.

The 30-year yield was up 1 basis point at 1.660 percent, also hitting a one-month high.

The markets will shift their attention to the two-day BOJ policy meeting, which starts on Wednesday.