The Adam Smith Institute declared May 30 "Tax Freedom Day" - a day later than last year and the latest it has fallen in the calendar since 2006.
Tory MP Steve Baker said it showed the "shocking truth about taxes and overspending".
He said: "This doomsday machine of deficit spending, debt and currency debasement will eventually blow up and there is no kindness in pretending otherwise. Politicians who are serious about the prosperity of our country and the well-being of the poorest within it should take note."
The date is calculated by measuring taxes and national insurance contributions as a proportion of the UK's net national income - producing a figure of 41.5% or 150 days as a share of the year. In France, the equivalent date does not fall until July but in the United States and Australia it comes as early as mid-April.
Institute director Eamonn Butler said: "Tax Freedom Day, which the Adam Smith Institute has been calculating for 25 years, is the plainest way to show what the tax burden really is. That is why the Treasury hates it. They of course want to conceal how much tax we pay, which is why they are so keen on stealth taxes.
"In the Middle Ages a serf only had to work four months of the year for the feudal landlord, whereas in modern Britain people have to toil five months for Osborne's tax gatherers.
"An increasing number of economists believe that Britain's taxes are too high and are choking off recovery. Some politicians say they need to keep taxes high in order to balance the Government's books.
"But the trouble with governments is that they always spend everything they raise in tax - and then as much more as they can get away with through borrowing. Just as the rest of us have had to cut back, so should the Government. The UK economy would be a lot healthier for it."