* Dollar/yen down 0.5 pct after hitting high of Y90.25
* Traders winding back yen selling ahead of BOJ decisions
* Markets position for aggressive action from BOJ
* Sterling hurt by weak British retail sales data
TOKYO, Jan 21 (Reuters) - The yen plunged into a fresh 2-1/2 year low against the dollar before quickly bouncing back on Monday as traders nervously braced for a Bank of Japan meeting that could see the central bank commit to an aggressive reflationary policy.
Facing relentless political pressure to pull the country out of deflation, the BOJ is expected to unveil a raft of policy steps including a 2 percent inflation target at its two-day meeting that ends on Tuesday.
The dollar rose to as high as 90.25 yen in early trade, edging past the previous high around 90.21 set Friday. But as traders rushed to lock in gains on caution ahead of the BOJ's policy meeting, it slipped to 89.70, 0.5 percent below its late U.S. levels last week.
Since mid-November, the dollar has risen about 13 percent on the yen.
"From here we will probably see rallies sold into in euro/yen, Aussie/yen etc as the street reduces short yen positions into the BOJ meeting tomorrow," said Jeffrey Halley, FX trader for Saxo Capital Markets in Singapore.
The euro bought 119.50 yen, off a 20-month peak of 120.73 last week while the Australian dollar shed 0.3 percent to 94.40 yen, slipping from a four-year high of 95.02 set Friday.
"We think the whole hope on the BOJ's policy will be peaking out soon barring any surprises tomorrow," said Minori Uchida, chief currency analyst at the Bank of Tokyo-Mitsubishi UFJ.
"The dollar could rise to as high as 91 yen but it will eventually return to 80-85 yen," he added.
Market expectations on the BOJ meeting are unusually high. Traders say financial markets are expecting a joint statement with the government to target two percent inflation, an increase in asset purchases with an open-end commitment, and possibly other measures such as cutting interest rates on excess reserves.
Data last Friday showed currency speculators slightly trimmed their bets against the yen in the week to Jan. 15, although they remained overwhelmingly negative on the currency.
Many market players remain pessimistic on the yen.
"We expect the door for further easing will likely be left open irrespective of the outcome of BOJ policy meeting, either explicitly by the BOJ or implicitly through government's plan to nominate doves to replace the governor and deputy governors," analysts at Barclays Capital wrote in a client note dated Jan. 20.
"Expectations of an aggressive monetary policy stance under new BOJ leadership are likely to provide support for USD/JPY in coming weeks. A post-announcement dip, if any, would provide better a entry level to go long USD/JPY."
Traders said there has been strong demand for options betting on further yen weakness, with one-month dollar/yen implied volatility - a measure of expected price movement - rising to its highest since August 2011 on Friday.
One-month risk reversals showed rising demand for yen puts, or bets on the yen falling.
With all eyes on the BOJ, other currencies took somewhat of a backseat.
The euro stood flat at $1.3323, having been capped by the $1.3400 level in the past week and facing strong resistance just under $1.3500.
The single currency showed muted response to a victory of Germany's centre-left opposition in a regional vote in Lower Saxony on Sunday. The euro crisis did not play much of a role in the vote.
Sterling slipped to a two-month low of $1.5838, remaining under pressure after an unexpected fall in retail sales last month raised the likelihood that Britain was slipping into its third recession in four years.
The Australian dollar, which came under a bit of profit taking late last week, continued to see good support under $1.0500. It was last at $1.0507, having bounced off a 1-1/2 week low of $1.0485 Friday.