HM Revenue & Customs has apologised after an online calculator designed to show users how much tax they were paying and how it was being spent crashed under the weight of traffic.

It said 400,000 people tried to access the calculator on the morning of its launch, far more than expected. Many received an error message telling them the calculator was not available.

A spokesman said it was hoped the tool would be operational within hours. The app version was not affected.

The calculator is part of HMRC's attempts to improve transparency in the tax system, which will lead to personalised tax letters being sent in 2014.