The probability that the Bank of Japan will intervene in foreign-exchange markets for the first time since March 2004 is at a six-year high of 51 percent, according to a Morgan Stanley model.

The strengthening of the yen and its misalignment from economic fundamentals is increasingly likely to spur a government response, Stephen Hull, London-based head of global currency strategy at Morgan Stanley, said Thursday.

Policymakers are facing pressure to aid the economy after the yen's climb to a 15-year high of 84.73 against the dollar on Aug. 11 stoked concern that exporters' earnings could weaken and deflation deepen. Trade minister Masayuki Naoshima said Thursday that the yen is too strong against the dollar and should drop about 6 percent to help exporters.

Japan hasn't intervened in the currency markets since March 16, 2004, when the yen was at about 109 per dollar. The BOJ sold ¥14.8 trillion during the first three months of 2004, after record sales of ¥20.4 trillion in 2003. The currency ended 2004 at 102.63 to the dollar.

Japan last bought the currency in 1998, purchasing ¥3.05 trillion as the exchange rate weakened to ¥147.66. The yen was trading around 85 per dollar Friday.