Gov. Bob McDonnell vowed Thursday to make the largest contribution to the public employee pension fund in Virginia history just days before the Republican leader presents his biennial budget.

The announcement comes on the heels of a state audit that shows the General Assembly has shorted the Virginia Retirement System by $20 billion, including by $8 billion in the past two years alone. McDonnell plans to recommend a $2.2 billion employer contribution to the retirement fund for teachers and state workers in fiscal years 2013 and 2014 to help close the gap.