A freedom of information request from accountants UHY Hacker Young revealed HMRC assessed more than 10,000 valuations in 2011 and gained an average of more than £27,000 in extra tax on each case.

The average the previous year was £24,600 a case, the Financial Times reported.

HMRC said challenging property valuations for IHT purposes has always been part of its work, and a spokesman for UHY said HMRC was under pressure to maximise revenues.

“For a bereaved family, it is unwelcome news to hear that HMRC wants to challenge your valuation of the estate and take even more of it in IHT,” Mark Giddens, partner, said.