Average earnings for most dentists are pounds 98,000 a year, reduced to a more modest pre-tax pounds 40,000 after taking account of practice expenses.
The Department said it believed the dentist had six assistants, but ministers are concerned about high earnings and are instituting checks.
The figures also alarmed Gordon Prentice, the MP for Pendle, Lancashire, who tabled the question after reading that one London dentist had been ordered to be retrained after carrying out unnecessary work, which left a woman patient with fillings in every tooth in her head.
"There is a can of worms here which needs to be looked at," said Mr Prentice. "I take the view that there should be a criminal offence for unnecessary treatment, which is the same as an assault. It is not enough for them to be retrained. They should be behind bloody bars."
Mr Prentice has tabled further questions to find out in which town Britain's richest NHS dentist lives, but so far the department has proved coy. "The town where I live [Barnoldswick] doesn't have a single NHS dentist and it has over 10,000 people," he said.Reuse content