Changes to income tax and national insurance will hit the poorest hardest, according to official Treasury figures out today.

For the first time, the Government's own figures have shown that the new system, announced by Gordon Brown in the March Budget report and due to take effect in April 2009, will penalise low-income earners.

The Conservative party today branded the overhaul a "con-trick", because it saif that at the time, the changes were presented as a tax cut for all.

Although Mr Brown announced a reduction to the basic rate of income tax by 2 pence, scrapping the 10 pence band, which benefitted lower earners cancelled out the beneficial effects for some taxpayers.

While parents and those earning more than £20,000 a year will benefit, single under 25 year-olds earning less than £20,000 will lose out by up to £200 a year.

A person under 25 on a salary of £10,000 will lose £197 a year. Someone earning £15,000 regardless of their age will lose £97 a year, according to the figures.

However a single person on a higher income of £35,000 stands to gain £303 a year.

Vince Cable, acting leader for the Liberal Democrat Party, said: "This confirms what we have always said, the Government’s tax policies have hit the poorest hardest... The public will be shocked that the lowest paid workers are the biggest losers from a policy that was supposed to help them. Taken together with the unfair council tax, we have a Government that talks of fairness and social justice but actually promotes policies which are regressive and favour the rich."

A Treasury spokesman said: "Once the Budget reforms are implemented, households will on average be £1,050 a year better off and families with children will be £1,800 better off in real terms than under the 1997 tax and benefit system.”