Mr Blanchard maintained that IMF calculations show "some decrease in debt, whether through haircuts or long rescheduling, is absolutely needed".
Should the Fund fail to gain guarantees that Greek debt is sustainable, it is poised to withdraw its involvement altogether.
During his final months as chief economist, Mr Blanchard made two personal interventions in blog posts which called for mass debt relief at the height of Greece's woes. He said he was motivated to do so because the IMF's position on Greece was being "misrepresented".
But European creditors are set to ignore the Fund's recommendations for repayment extensions of up to 40 years. They will instead propose to "cap" the amount the government pays to reduce its debt to 15pc of GDP a year.