Former Greek prime minister George Papandreou has said it would be a "real crime" if a debt financing deal is not agreed with European leaders.
He insisted it was time to move away from the "acrimony" and focus on the issues of "deeper reforms".
A grace period would allow Greece to commit to these, he added.
Mr Papandreou told BBC Radio 4's Today programme: "After five years of efforts from the Greek people, huge sacrifices, it would be a real crime not to be able to finish this deal and finish the programme and hopefully access the markets as soon as possible to stabilise Greece.
"Europe has got the possibilities to help in many ways.
"I think we need to move out from the antics of acrimony and really look at the issues. I would say let's focus on the issues of deeper reforms rather than future cuts and painful budgetary issues."
Asked if other countries ultimately have to accept Greece will never be able to pay everything back, he replied: "I would say, yes."
He suggested a referendum would be a good way of asking the Greek people what they want.
European leaders want an agreement secured over the weekend, just days before Athens has to meet a crucial repayment deadline that carries the risk of bankruptcy and euro-exit.
A deal on a draconian austerity package is vital for creditors to unfreeze 7.2 billion euro (£5.1 billion) in bailout money that would get prime minister Alexis Tsipras off the hook for the time being.
Yesterday, a meeting of eurozone finance ministers broke up over disagreement on the Greek rescue package, intensifying doubts about whether Athens can pay the International Monetary Fund debts worth 1.6 billion euro (£1.1 billion) on Tuesday.
Greece's left-wing government and creditors have so far fallen short of a deal after leaders from the IMF, the European Central Bank and the European Commission raised the stakes by putting forward a joint set of demands from Greece.