Hardeep Singh Nijjar was shot dead outside a Sikh temple on 18 June in British Columbia.

Mr Trudeau said Canadian intelligence has identified a "credible" link between his death and the Indian state.

India called the claim "absurd". Mr Trudeau raised the issue with Indian Prime Minister Narendra Modi at the recent G20 summit.

"Any involvement of a foreign government in the killing of a Canadian citizen on Canadian soil is an unacceptable violation of our sovereignty," Mr Trudeau said on Monday in the House of Commons.

"It is contrary to the fundamental rules by which free, open and democratic societies conduct themselves."

India has previously denied any involvement with Mr Nijjar's murder.

On Tuesday, India's Ministry of External Affairs said in a statement that it rejected Mr Trudeau's claims.

"Allegations of government of India's involvement in any act of violence in Canada are absurd and motivated," said the ministry.

"Similar allegations were made by the Canadian prime minister to our prime minister, and were completely rejected."

Canada also expelled an Indian diplomat, Pavan Kumar Rai, on Monday over the case, Foreign Minister Melanie Joly told reporters following Mr Trudeau's remarks.

Ms Joly said Canadian officials are limited in what they can say in public about the case due to the ongoing homicide investigation into Mr Nijjar's death.

Investigators have previously categorised the death of 45-year-old Mr Nijjar as a "targeted incident".

Mr Nijjar was shot dead in his vehicle by two masked gunmen on a mid-June evening in the busy car park of the Guru Nanak Sikh Gurdwara in Surrey, a city about 30km (18 miles) east of Vancouver.

 Hardeep Singh Nijjar was murdered on 18 June in Surrey, British Columbia, in what police have described as a "targeted" attack

He was a prominent Sikh leader in the western-most province of British Columbia and publicly campaigned for Khalistan - the creation of an independent Sikh homeland in the Punjab region of India. His supporters have said that he was a target of threats in the past because of his activism.

India has previously described him as a terrorist who led a militant separatist group - accusations his supporters call "unfounded".

Mr Trudeau said Canada had expressed its concerns about Mr Nijjar's death to high-level security and intelligence agencies in India.

He also raised it with US President Joe Biden and UK Prime Minister Rishi Sunak.

"I continue to ask with a great deal of firmness that the government of India co-operate with Canada to shed light on this situation," he said.

Mr Trudeau said that Mr Nijjar's shooting has angered Canadians, leaving some fearful for their safety.

Moninder Singh called for a public inquiry into the murder

After Mr Trudeau's comments in Ottawa, several large posters and tributes to Mr Nijjar were visible at the Guru Nanak Sikh Gurdwara in Surrey.

Moninder Singh, a spokesman for the British Columbia Sikhs Gurdwaras Council, told the BBC there was a sense both of frustration and appreciation in the community after Mr Trudeau's comments.

"This operation being allowed to be carried out on foreign soil in Canada," he said. "That's where the frustration comes from.

"The appreciation comes from the fact that at least the prime minister stood up and acknowledged that there is a foreign hand behind this murder, and this assassination."

Other Sikh groups in Canada, including the World Sikh Organisation, welcomed the prime minister's statement, saying Mr Trudeau confirmed what was already widely believed in the community.

 Mr Trudeau had a tense meeting with Mr Modi last week during the G20 summit in India

There are an estimated 1.4 to 1.8 million Canadians of Indian origin. The country has the largest population of Sikhs outside the state of Punjab in India.

Mr Trudeau's remarks come after his tense meeting with Mr Modi last week during the G20 summit in India.

During that meeting, according to a statement from the Indian government, Mr Modi accused Canada of not doing enough to quell "anti-India activities of extremist elements", referring to the Sikh separatist movement in the country.

Canada also recently suspended negotiations for a free trade agreement with India. It gave few details on why, but India cited "certain political developments".

Mr Nijjar is the third prominent Sikh figure to have died unexpectedly in recent months.

In the UK, Avtar Singh Khanda, who was said to be the head of the Khalistan Liberation Force, died in Birmingham in June under what have been described as "mysterious circumstances".

Paramjit Singh Panjwar, who was designated a terrorist by India, was shot dead in May in Lahore, the capital of Pakistan's Punjab province.

The backdrop to the tension between Delhi and Ottawa is the increasing pressure the Indian administration has put on governments of three countries with sizeable Sikh populations: Canada, Australia and the UK.

It has openly said that a failure to tackle what it calls "Sikh extremism" would be an obstacle to good relations.

Australian officials said they would look into vandalism of Hindu temples by pro-Khalistan activists, but would not stop Australian Sikhs expressing their views on an independent homeland.

Mr Johal has been held in an Indian prison for more than six years without trial, accused of extremist activities, but he says he has been tortured and forced to sign a confession.

Human rights group Reprieve says it has evidence that his arrest came (while he was in India to get married) after a tip-off from British intelligence.

By Nadine Yousif & Aleem Maqbool