The taxman says three and a half million people are due a refund, but two million will have to fork out for underpaid tax.
HMRC tax calculator website crashes
The Treasury's mobile tax app crashed hours after its launch
A website aimed at showing taxpayers how the Government spends their money has crashed hours after its launch.
The HM Revenue and Customs (HMRC) tax calculator was supposed to allow users to work out how much tax they can expect to pay and how it will be distributed.
But curious internet users hoping to access the website were faced with an apology as the page embarrassingly buckled under the pressure of 400,000 hits amid unprecedented demand.
The message said: "Sorry, the HMRC Tax Calculator is currently not available. We apologise for any inconvenience caused. Please try again later."
A spokesman said: "There are thousands and thousands of people trying to use it. It's slow to get going. It's purely because of the phenomenal demand."
The fault came only hours after Exchequer Secretary to the Treasury David Gauke unveiled the initiative, saying: "We think it is right that people know how much tax they pay and what the Government spends it on.
"At the moment, over half of taxpayers do not know how much income tax and National Insurance they pay in a year.
"Our new tax calculator will allow them to find out, which is a big step towards a more transparent, 21st century system."
The calculator was launched as a mobile phone app downloadable from the Apple Store and Google Play, as well as being available from the HMRC website.
The innovation is part of a drive by Chancellor George Osborne to ensure taxpayers have a better idea about how ministers use their money.
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