Twenty developing countries have called on rich nations to scrap their farming subsidies within five years.
The G20 ministers are keen to present a united front
Brazilian Trade and Foreign Minister Celso Amorim, who heads the group known as the G20, said the handouts were "the most harmful single piece of commerce".
He was speaking on Saturday after a G20 meeting in the Indian capital Delhi.
The bloc approved a common negotiating position ahead of a key ministerial conference of the World Trade Organisation (WTO) in December.
Developing nations accuse rich countries of driving down the price of produce worldwide through subsidies and protecting their own markets.
"We have called for substantial reductions in subsidies given by rich countries to their farmers," Mr Amorim said.
The United States, European Union and Japan have offered to reform their agricultural policies, but developing countries do not think they go far enough.
The 25-member EU - which gives the most generous breaks to its farmers - offered last year to phase out its subsidies but did not set a deadline.
"This waffling on subsidies must end. The EU and US cannot offer us post-dated cheques," Indian Trade Minister Kamal Nath said.
"They have to eliminate export subsidies as fast as possible and no later than five years," he added.
The G20 was formed 18 months ago at the time of the failed Cancun trade summit in Mexico.
The most powerful members are India, China, Argentina and Brazil, which have substantial farming sectors.