The Institute of Fiscal Studies has warned that taxes will have to rise under the current budgetary rules.
The IFS calculates that taxes will have to rise by £8 billion by 2013 in order for the Chancellor not to break the borrowing and spending plans he has imposed.
Between 2008 and 2010 borrowing will have to rise by £40?billion in each year, it says.
The institute, which uses the Treasury's own figures in its calculations, says that while the £8 billion tax rise - equivalent to around 2p on income tax - would be "prudent", the Government is likely to shy away from it, forcing it to borrow even more or spend less.
According to the report, by 2012 families will earn £5,500 more on average, but £2,900 of this increase would be taken in taxes if the Chancellor was to stay within his targets.
This should give sufficient warning of the government's untenable budgetary position, especially if Northern Rock's debt is added. This is a stark warning to George Osborne: cut spending and taxes.