Millions of households across the UK are set to receive letters from HM Revenue and Customs (HMRC) this summer as the tax authority begins issuing around four million income tax refund notices.
HMRC started sending the letters on June 1, with the mailout scheduled to continue until the end of August.
The tax authority warned recipients they must respond if they are asked to provide additional information in order to receive any repayment owed to them.
Officials said: "You must respond to the letter so we can make any repayments owed to you as soon as possible."
HMRC figures showed that almost one million people failed to claim tax refunds last year because they did not respond to correspondence from the department.
The average unclaimed repayment stood at £473, according to the tax authority.
Refunds are typically issued when someone has paid too much income tax during the financial year, often because they were placed on the wrong tax code, changed jobs or worked multiple jobs simultaneously.
Once the tax year concludes in April, HMRC reviews records to determine whether taxpayers have overpaid and then issues what is known as a P800 letter to those eligible for a repayment.
HMRC tax refund letters sent to millions as Britons warned not to ignore post
|GETTY
Officials said: "Last year, almost 1 million people didn't claim back the money they're owed."
They added: "On average people get £473 back when claiming a tax refund."
Taxpayers who choose to claim their repayment digitally through their Personal Tax Account or the HMRC app are likely to receive their money within five working days.
Those opting to receive payment by cheque could wait up to six weeks for the funds to arrive.
Although many refunds are processed automatically through the P800 system, some people will still need to submit claims manually depending on their circumstances.
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Some people may need to submit their claims manually
| GETTYThis can include workers claiming tax relief on employment expenses such as home working costs, specialist equipment or other work-related spending.
People are also able to backdate claims for up to four previous tax years using the HMRC app or forms available through GOV.UK.
HMRC separately warned that fraudsters are attempting to exploit the refund season by sending fake messages and emails to taxpayers.
The department said members of the public should remain cautious if they receive requests for further information from suspicious email addresses.
Officials said: "If you receive further requests for information to be sent to an email address that does not end in 'hmrc.gov.uk', they will not be genuine and you should report this to HMRC."
Genuine HMRC correspondence may still ask taxpayers to provide further details to support repayment claims or verify information linked to refunds.
However, the tax authority stressed that any legitimate email communication will always come from an address ending in "hmrc.gov.uk".
HMRC has repeatedly urged taxpayers to avoid clicking on suspicious links or sharing personal information with unknown senders amid continuing concerns about scam activity targeting households across Britain.