W 50p tax rate for top earners has been triggered as the new financial year begins.
The new income tax rate on taxable incomes above a £150,000 has been attacked by business leaders who claim it will dent business confidence and stifle entrepreneurial spirit,
The Institute of Directors also warned that foreign investors could stay clear of the UK.
Meanwhile CBI president Martin Broughton said the tax was "economic vandalism".
But the government claims the tax rate – announced in the 2009 budget – was a necessary step to help fill the hole in public finances.
In other changes, the new tax year has also seen child tax credit increased by £20 a year.
The amount of money that people aged under 50 can save in an ISA has also risen from £7,200 to £10,200 in a financial year.
Half of this can be saved in cash, with half, or all, in stocks and shares.