A fifth of all senior federal judges including former Chief Justice Warren E. Burger stand to collect their largest pay raises in a decade even though they no longer handle any court cases, The Associated Press has learned.

This windfall - increasing Burger's salary by $60,000 to $175,000 a year - stems from a retirement system allowing federal judges to reduce their caseloads sharply while still qualifying for active-duty pay increases. In stepping down from full-time duty, they accept a title of "senior judge."Aside from Burger, the 305 other senior judges also stand to see their salaries rise by about 50 percent unless Congress disapproves the raises due for top federal officials on Feb. 8.

Judges who retire through resignation - the other option provided by Congress - keep for life the same salary they received on their last day of work. In Burger's case, it would have been $108,400.

Senior status is the vastly more popular option. Burger, for example, already has received one raise, in 1987, to $115,000.

Administrative records obtained by the AP indicate that roughly one in five of the nation's senior judges do no judicial work whatsoever. They get the raises but are as hard to find on the bench as the 13 former judges who chose full retirement.

"It doesn't seem fair," said Rep. Carlos Moorhead, a member of a House subcommittee overseeing the federal judiciary.