A tribunal has ordered that secret Downing Street files relating to Anglo-Indian relations at the time of the 1984 massacre at the Golden Temple of Amritsar must be made public.

Campaigners say the Margaret Thatcher-era documents could reveal further information about the UK’s military role in the deaths of hundreds and possibly thousands at Sikhism’s holiest site following a violent assault by the Indian army in June 1984.

The information tribunal said this week there was “a high public interest” in disclosure – partly in response to the “strength of feeling of the Sikh community in the UK and beyond” – and set aside objections from the Foreign Office, which said declassification could adversely affect the UK’s relations with India.

The decision, which followed an appeal brought by journalist Phil Miller, is the latest step in a lengthy disclosure battle that began in 2014 after it emerged that an SAS officer had been dispatched in February 1984 with the approval of Thatcher to advise on Indian army plans to remove dissident Sikhs occupying the temple.

David Cameron, then prime minister, immediately ordered an inquiry by the cabinet secretary, Sir Jeremy Heywood, who examined government files and concluded that UK involvement was limited to the visit by the SAS officer. However, the official files were not made public as part of the Heywood review and there have since been legal attempts to force their disclosure.

Bhai Amrik Singh, the chair of the Sikh Federation in the UK, said the judgment “confirms the Heywood review was limited and will add to the evidence we have already presented to prove it was a whitewash”. Singh called on Theresa May to consider holding a public inquiry. He said the prime minister “should not listen to those paranoid about our relations with India”.

An Indian Sikh devotee takes a dip in the holy sarovar at the Golden Temple. Photograph: Narinder Nanu/AFP/Getty Images

The Downing Street files that the government has been ordered to declassify include one that covers UK-Indian relations between March 1984 and May 1985, and another that relates to the assassination of Indira Gandhi in October 1984 by some of her Sikh bodyguards, and Thatcher’s visit to India to attend her funeral.

Labour is supporting the legal challenge and called on the government to release the files promptly. The Cabinet Office has a month to decide whether it will appeal against the ruling, and said that it was currently considering its position.

Labour’s deputy leader, Tom Watson, said: “It has been a source of deep hurt to the British Sikh community for the past four years that Britain could have played a role in the terrible massacre of 1984. If the government appeals against this decision it will be a cowardly attempt to hide the truth.”

The unanimous judgment by the tribunal led by Judge Shanks records that two Foreign Office officials gave evidence in the hearing, often in secret session. Owen Jenkins, a former director of the FCO’s south Asia and Afghanistan desk, unsuccessfully warned that “the release of some of the withheld documents would have damaged relations with India by showing that the UK government did not regard the activities of the Sikh extremists with sufficient concern and was ‘soft’ on them”.

But in the judgment, the tribunal disagreed and the documents “tend to show how seriously the issues were take at senior levels in the UK government”.

The tribunal also ruled that one other set of files, relating to intelligence assessments prepared for the joint intelligence committee had to remain secret because they were covered by the blanket freedom of information exemptions that apply to the UK’s spy agencies.