The tax band had originally been due to rise to 45 percent from 40 percent in April 2011 as Britain seeks to claw back lost tax revenue caused by a deep recession.
The 50-percent rate will apply to any income above 150,000 pounds.
"In November, I announced a new rate of income tax of 45 percent on incomes above 150,000 pounds -- the top one percent of taxpayers," Darling said in his budget speech.
"In order to help pay for additional support for people now, I have decided that the new rate will be 50 percent and will come in from next April -- a year earlier."