The Indian government also criticised Ottawa for expelling one of its diplomats, and dismissed allegations that it was linked to the killing of the activist in Canada earlier this year as "absurd".

"The decision reflects the government of India's growing concern at the interference of Canadian diplomats in our internal matters and their involvement in anti-India activities," India's Ministry of External Affairs said in a statement.

"The concerned diplomat has been asked to leave India within the next five days."

It comes after Prime Minister Justin Trudeau said there were "credible allegations" that the Indian government was behind the assassination of Mr Nijjar earlier this year.

The Canadian citizen was shot dead outside a Sikh temple in Surrey, British Columbia, on 18 June.

He had been a prominent supporter of the Khalistani separatist movement, which wants to establish an independent homeland for Sikhs in India, where they are a minority.

Mr Nijjar was organising an unofficial referendum in India for an independent Sikh nation at the time of this death

Alongside Mr Trudeau's comments, Canadian foreign minister Mélanie Joly said the head of Indian intelligence in Canada had been expelled over the allegations.

The Indian government said it "completely rejected" the allegations, and added: "We are a democratic polity with a strong commitment to rule of law.

"Such unsubstantiated allegations seek to shift the focus from Khalistani terrorists and extremists, who have been provided shelter in Canada and continue to threaten India's sovereignty and territorial integrity.

"The inaction of the Canadian Government on this matter has been a long-standing and continuing concern.

"We reject any attempts to connect the government of India to such developments."

The duelling expulsions come amid tense relations between Canada and India - who are both members of the G20.

Trade talks between the two countries have derailed, while Canada has cancelled a trade mission to India which was scheduled for the autumn.

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Mr Trudeau told parliament that he had brought up the killing with Indian Prime Minister Narendra Modi at the G20 last week.

He also said he had told Mr Modi that any Indian government involvement would be unacceptable and that he had asked for cooperation in the investigation.

On Tuesday, a spokesperson for UK prime minister Rishi Sunak, responding to the news, said it was "vitally important" that sovereignty and international rule of law was respected.

However, the spokesperson said that the UK government was not looking to "pre-empty investigations".

The spokesperson also said work on a trade deal with India would continue.