India has lifted a decades-old ban on civil servants being members of a far-right Hindu nationalist group that is the ideological parent of Prime Minister Narendra Modi's ruling party, according to a government memo made public on Monday.

Several top leaders of Modi's Bharatiya Janata Party (BJP), including the PM himself, cut their teeth at the Rashtriya Swayamsevak Sangh (RSS), whose millions of members conduct paramilitary drills and prayer meetings.

The organisation campaigns for India to be declared a Hindu nation -- rather than a secular one, as enshrined in its constitution -- and critics accuse it of fomenting sectarian divisions.

Since 1966, Indian civil service rules have banned government employees from membership or participation in the activities of the RSS and the Islamist organisation Jamaat-e-Islami.

But a new official memorandum issued earlier this month and made public on Monday by BJP publicity chief Amit Malviya stated: "It has been decided to remove the mention" of RSS from the rule.

In a post on X, Malviya said the rule was an "unconstitutional order issued 58 years ago".

The government memorandum did not mention Jamaat-e-Islami.

Opposition leaders lashed out at the amendment.

Jairam Ramesh of the Congress party said "the bureaucracy can now come in knickers" -- a reference to the khaki shorts that RSS members wore as part of their uniform until recently.

The RSS has briefly been declared an illegal group on three separate occasions, including after the assassination of independence hero Mahatma Gandhi by a former member.