By Matt Falloon

LONDON (Reuters) - The government may be overestimating the benefits of a higher tax on the wealthy and big cuts in spending lie ahead to get public finances back on track, a leading thinktank said on Tuesday.

Chancellor Alistair Darling on Monday unveiled a 45 percent rate for those earning above 150,000 pounds from April 2011 as part of measures intended to help recoup some of the cost of supporting the economy during the downturn.

Darling laid out plans for a 20 billion pound stimulus package to support the economy but also ramped up his borrowing forecasts considerably.

"The central estimates in our paper would leave you to conclude that it (the new income tax rate) would raise approximately no money -- it might raise a small amount of money, it might lose a small amount," said Mike Brewer, director of tax and welfare research at the Institute for Fiscal Studies.

The Treasury estimates the new tax rate -- which will take effect after the next election and marks the abandonment of a long-held Labour pledge not to raise taxes on high earners -- would eventually raise 1.6 billion pounds a year.

Brewer said it was hard to gauge to what extent the wealthy could use loopholes or even emigrate to avoid the new tax.

"We stand by our costings," a Treasury spokesman said, arguing that tax avoidance measures and limits on pension contributions would reduce the extent to which high earners could shift money away from the tax net.

The IFS said little of Darling's plan to bring the public finances back into line relied on tax rises.  Continued...