Up to 3.5 million people are to receive a tax rebate this year, while more than 1.5 million will be hit with a tax bill.
Britons will begin to receive tax rebates in the next two weeks, with the average payout expected to be £379.
But up to 1.6 million people are to be told that they have not paid enough tax in the 2011-12 financial year and owe the public purse an average of £537.
HMRC is getting its tax affairs in order more quickly this year, with letters going out two months earlier than last year.
The first letters are to go out on May 17, with the last to be received in October later this year.
People have been told that they do not need to contact HMRC and any tax owed will be automatically paid to them without a claim needing to be made.
"We are pleased that we are able to start this process more quickly than in previous years, giving money back to those we owe and delivering certainty to those with something to pay," said Stephen Banyard, acting director general for personal tax.
HMRC said that people usually pay too much or too little tax when their circumstances change.
Examples included receiving benefits in kind from employers, having more than one job or having large periods between work.
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