Tablets given to several women who died after a mass sterilisation programme in India may have contained rat poison, it has emerged.

Early tests on a batch of the antibiotic Ciprocin, which was given out after the operations in Bilaspur city, found it contained zinc phosphide, chief administrator for the district, Siddhartha Pardeshi, told Reuters.

The compound is used as a rodenticide but is highly toxic to animals and humans who ingest it in sufficient doses.

At least 13 women who took the tablets have died and dozens more remain in hospital.

Most had taken part in a state-run sterilisation programme aimed at reducing India's ballooning population.

Some 83 women were paid £15 to undergo laparoscopic tubectomy operations at the family planning camp in Bilaspur, in Chhattisgarh state.

SK Mandal, the state's chief medical officer, said has said that post-mortem reports did not suggest any surgery faults in sterilisation procedures.

Samples of the drug the women were given have now been sent to laboratories in Delhi and Kolkata for further testing, Mr Pardeshi said.

He said he expected the tests to show that the drugs the women were given contained the poisonous compound.

"This is what we anticipate," he said.

"Symptoms shown by the patients also conform with zinc phosphide (poisoning)."

It had earlier emerged that a quantity of zinc phosphide was found at the nearby factory of Mahawar Pharmaceuticals, the firm at the centre of investigations.

Seizures

The state government said it had seized 200,000 tablets of Ciprocin 500 and more than 4 million other tablets manufactured by Mahawar.

Mahawar was barred from manufacturing medicines for 90 days in 2012 after it was found to have produced substandard drugs. It did not, however, lose its licence.

Mr Pardeshi and Chhattisgarh health minister Amar Agarwal said an investigation was under way into why the drugs were bought locally when there was a large enough stock of Ciprocin made by other makers held by the state's central procurement agency.

Mr Agarwal said: "There was no incentive to procure locally so we need to investigate why it was done. This means something is wrong."

Police have arrested Mahawar's managing director Ramesh Mahawar and his son. The company says both are innocent.

A doctor, RK Gupta, was also arrested earlier in the week over the deaths and remains in custody.

Gupta has denied responsibility for the deaths and blamed the drugs.

"I have been performing surgeries for a long time and there has never been any problem," Dr Gupta told reporters in Bilaspur as he was arrested.