Demands for underpaid tax are being sent to 1.2 million people(Image: Gareth Fuller - PA Wire)

More than a million people will receive letters from the taxman in the next three months demanding money.

The HMRC is contacting 1.2 million people about underpayments during the 2010/11 tax year, with the average unexpected tax bill likely to be around £550.

It was announced back in June that 4.7 million Brits had paid the wrong tax in 2010/11 - hardly a ringing endorsement of HMRC.

The fortunate 3.5 million who had overpaid have already been contacted, so now it is the turn of those who owe money.

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What to do if you're contacted
If you are among those who receive a bill, you do have the right to appeal, but the outcome will very much depend on your personal circumstances.

If your appeal is unsuccessful then it may be possible to have the tax deducted via a change to your 2012/13 tax code, thus spreading the burden over 12 months.

If you still could not afford that, discuss it with HMRC and it could be spread over two or more years.

How to challenge the taxman

Watch out for phishing attacks
Of course, this news is likely to alert criminals looking to carry out phishing attacks, so it's important to be vigilant.

Remember that the HMRC will only contact you by letter, phone or via your accountant. It will never contact you by email.

If you are in any doubt either check with your accountant or with HMRC, using only the details you have searched for yourself (ie not via a link or contact details in an email).

Related links
Tax relief: are you claiming all you can?
MSN Money's tax centre
Get a full tax review for £70