The Hindu Human Rights pressure group is opposing the involvement of the International Society for Krishna Consciousness - known as the Hare Krishna movement - in the planned Krishna-Avanti primary in Edgware, north-west London.
Campaigners describe the movement, known for its orange robes and chanting, as a new-age sect, which does not represent mainstream Hinduism. The faith has its roots in 1960s counter-culture, with famous followers such as the late Beatle George Harrison.
Arjun Malik, spokesman for Hindu Human Rights, said: "If there is to be a Hindu school, it should be run by a mainstream group, not a sect. It gives the Hindu community a bad name."
The school, due to open in 2010, has already run into trouble over plans to plant over five acres of greenery. It is being set up by the charity I-Foundation, which has links with Hare Krishna.
A spokesman rejects the cult tag, describing Hare Krishna as a "young and exciting" movement, comparable with Christianity's Alpha Course, whose followers explore the meaning of life.
But the row is likely to fuel the debate over faith schools, sparked by the expansion of Sir Peter Vardy's Emmanuel Schools Foundation, which critics say has links to the pro-creationist lobby.
Hindu Human Rights raised concerns about Hare Krishna-linked schools, in the United States, where 95 former pupils sued for child abuse in 2000.
Professor Eileen Barker, a specialist in new religion at the London School of Economics, said the movement has since "cleaned up its act" and purged itself of undesirable members.
But Ranbir Singh, chairman of Hindu Human Rights, said: "If the Mormons wanted to open a school here, people would ask questions. But in this case the government hasn't done its research."
Former devotees writing on the Cult Education Forum website describe the group as fanatical, high-pressure and insular, alleging it places too much power in the hands of a few gurus.
Gauri Das, president of the temple in Watford, said he was saddened by the attacks. "I remain confident that the Krishna-Avanti primary school will be a beacon to the community, and an inspiration to the next generation," he said.
In a letter to Hindu Human Rights, Alison Powell of Schools Capital Division at the Department for Education and Skills, said there had been a "clear demand" for the school in Harrow, where 20 per cent of the local population are Hindu.
From flower power to child abuse
The Hare Krishna movement is an evangelistic faith with a history of charismatic leaders. It focuses on the Hindu god Krishna, and is based on literal interpretations of ancient Hindu texts. Devotees abstain from meat, fish, eggs, gambling and alcohol.
The movement was founded in New York 40 years ago by AC Bhaktivedanta Swami Prabhupada, an Indian-born guru. It became popular with the 1960s hippy movement, and by the 1970s there were 100 temples worldwide and 3,000 full-time members.
Prabhupada's death in 1977 triggered internal strife: there were convictions for brainwashing, racketeering, fraud and murder.
In 2000, the organisation paid pound;5 million to more than 400 people who claimed to have been abused at 12 US Hare Krishna schools during the 1970s and 1980s. Leaders purged the organisation of those involved and set up a Child Protection Office.
Today there are 5,000 members in Watford, the largest UK temple. A mix of Hindu faiths attend.