Greenpeace has accused the Indian government of a systematic crackdown on its activities after one of the charity’s staff was turned back by immigration officials at Delhi airport and returned to the UK despite having a valid visa.

The decision to refuse British national Ben Hargreaves entry to attend a meeting follows a ban on donations from Greenpeace International being transferred into Greenpeace India’s account. The ban was introduced by prime minister Narendra Modi’s rightwing government in June.

The ban, which the Delhi high court ordered the government to lift this month, was imposed after a leaked Indian intelligence report described the organisation as “a threat to national economic security”.

Samit Aich, executive director of Greenpeace India, told the Guardian: “We have seen for the last couple of months a definite move to scuttle Greenpeace’s work in India by various ways and means. Ben had a valid visa and was unceremoniously told to go back [to England]. This is clearly a symptom of a systematic clampdown of on Greenpeace in India.”

Greenpeace claims the leaked intelligence report, which cited the charity’s protests against nuclear and coal plants and accused it of causing an annual reduction in India’s GDP of 2-3%, contained fabricated information, and says its funds were frozen without any explanation or evidence of wrongdoing.

Aich said the Indian government was unhappy that Greenpeace had stepped up its campaign to protect community rights and ancient forests in the Mahan region of Madhya Pradesh, where it says plans for a coal mine proposed by a UK-registered company, Essar, threaten people’s livelihoods and biodiversity.

Aich said the charity’s campaigning was validated by a ruling by India’s supreme court last month that all coal mining licences awarded since 1993, including the one covering Mahan, were illegal, amid claims of corruption, mismanagement and underpricing.

Hargreaves, who had previously travelled to India without any problems, was given no reason for his deportation, and Greenpeace said other British nationals from the charity had been detained and interrogated at Indian airports.

The charity’s UK executive director, John Sauven, has written to the foreign secretary, Philip Hammond, requesting an explanation for the treatment of Hargreaves and other British nationals from the Indian minister for external affairs.

“We fail to understand why Ben wasn’t allowed to return to India when he has all the appropriate permits,” said Sauven. “This is not the first time that our staff have been subjected to unfair treatment as they tried to enter the country – this is unacceptable. We are a legitimate part of global civil society. It’s very disturbing that the authorities in the world’s largest democracy should act in such an arbitrary way.”

The Indian high commission in London did not respond to the Guardian’s request for a comment.