Citing what they called detailed evidence, the signatories to the letter wrote that they are particularly concerned about the Prime Minister's relationship with the RSS which they described as a "paramilitary organisation modelled on the Italian Fascist and German Nazi parties known for its violent misogyny openly displayed in the pronouncements of its leaders."
The letter also states that the overseas wing of the RSS is currently being investigated by the British Charity Commission for hate speech against Christians and Muslims.
Referring to right-wing organisations, the letter states, "Surely these are not organisations by which a Prime Minister of democratic and secular India should be guided, or held accountable."
"These organisations are also responsible for vicious 'moral policing' which has led to murderous attacks on couples who cross religious and caste boundaries. Your silence sends them a message of approval," it said.
More From This Section
The letter cites many instances of violence against women, including during the 2002 Gujarat riots, in which justice is still awaited.
Modi is scheduled to visit the UK in the second week of November.