From the IFS Wiki page:
The Institute frequently speaks out on politically important issues and has at different times been criticised from both sides of the political spectrum. In October 2010, Deputy Prime Minister Nick Clegg accused the IFS of using methods that were "distorted and a complete nonsense" [12] after it challenged government claims that tax and benefit reforms in the June 2010 Budget were "progressive." [13] Left-wing think tank Tax Research UK stated in a report that the "Institute for Fiscal Studies is a body that persistently recommends tax increases that benefit the wealthiest in society at cost to those who make their living from work and the poorest in society".[14] On another occasion, the right-leaning magazine The Spectator published a leader stating that "‘institutes’ funded by research grants (which means, usually, tax money) will always argue for more expensive meddling by the state" and that the Institute for Fiscal Studies was "the most striking example" of this.[15]
Not really a surprise seeing who benefits from their 'forecasts', is it?