The lower house of the Indian national assembly, the Lok Sabha, has approved a controversial £13bn plan to provide cheap grain to the poor – a key part of the ruling Congress party's strategy to win re-election next spring.

Under the plan, the government will sell subsidised wheat and rice to more than two-thirds of its 1.2 billion population.

India is home to a quarter of the world's hungry, according to United Nations data, despite being one of the biggest food producers and experiencing years of rapid economic growth.

The vote broke a long stalemate in parliament, potentially clearing the way for several reforms aimed at spurring the flagging economy, which the government hopes to pass in an extended session that ends in two weeks. The upper house – the Rajya Sabha – must approve the decree before it becomes law.

India's main opposition party, the Bharatiya Janata party, has criticised the welfare scheme, which expands an existing cheap food programme covering more than 200 million people, as still too narrow to tackle widespread malnutrition among India's poor. The country's central bank has warned that increased spending could deepen the government deficit and boost inflation.

However, the party voted for the bill, which was passed on Monday evening after nearly nine hours of debate and the inclusion of amendments that government sources say could lead to an additional requirement of about 3m tonnes of grain.

Alarm over the government deficit caused stock markets to plunge and pushed the already weakened rupee to a new record low on Tuesday.

Manmohan Singh, the prime minister, has called child malnutrition in India, where almost 50% of children are underweight, a "national shame". But critics have said the plan merely expands a wasteful and inefficient public distribution system at a time when public finances are under huge stress, as global and local investors lose faith in India's potential to grow fast in the immediate future.

The rupee has plunged against foreign currencies in recent weeks, while India still imports vast quantities of oil and gold. Some say widespread reform of the agricultural sector would lead to lower food prices and boost growth.

The bill is a pet project of Sonia Gandhi, who led the Congress party to victory in the last two elections on the back of populist programmes such as a rural jobs plan and a multibillion-pound farmer loan waiver passed just before the most recent general election in 2009.

Congress has suffered from a series of corruption scandals, bitter internal feuding and its apparent inability to tackle the deep economic and social challenges facing India.

Gandhi, 66, was taken to hospital at the end of the debate, suffering from viral fever, but was later discharged after being given the all-clear.