Rajyasri Rao BBC correspondent in Delhi

The spread of industry is believed to be a factor in the contamination

Nearly three-quarters of spinach samples tested by a joint Indian-UK team exceeded Indian safety standards, the head of Indian non-government organisation Toxics Link told the BBC.

The report warns that traditional cooking methods do not reduce contamination levels.

It comes as the Indian Government is planning new restrictions on the use of pesticides in bottled water.

Researchers surprised

Spinach is a hugely popular seasonal vegetable, even among poor families.

In comparison to the European Union, our standards are far less stringent Ravi Agarwal Toxics Link

But a study jointly conducted by the Toxics Link environmental organisation and Imperial College in London casts doubts over its safety.

The Delhi head of Toxics Link, Ravi Agarwal, told the BBC that researchers were surprised by the results.

"We did of course suspect to find toxicity but the percentage by which it crosses the designated safe mark has left us surprised."

The percentage of lead should not exceed 2.5 milligram per kilogram according to Indian national safety regulations.

But up to 73% of spinach samples studied failed to meet this benchmark.

"In comparison to standards set by the European Union, our standards are far less stringent," Mr Agarwal said.

"So the fact that such a large part of the samples failed to meet even national toxicity limits is worrisome."

It is hard to pinpoint any one reason for these high levels.

But three factors are seen as contributing:

• Highly contaminated air and water in and around Delhi

• The increasing spread of industrial units toward farm lands in outlying areas where the spinach is grown

• The overuse of pesticides on crops

The good news is that washing spinach leaves at least twice can help halve toxicity levels.

But the bad news is that boiling the leaves in water before cooking it, as most Indian families do in order to "burn up" unpalatable foreign objects, does not remove heavy metals like lead.

Analysts say that since the toxicity is rooted in the commercial practices of the city, it will take tremendous changes in Delhi's economy to help make spinach fully safe to eat.

In February it emerged that the levels of pesticides in bottled water in India was unacceptably high.