As Parliament debated a bill that would give new powers to Canada’s police and spy agencies, four former prime ministers called on the government to simultaneously strengthen their oversight.

“The lack of a robust and integrated accountability regime for Canada’s national security agencies makes it difficult to meaningfully assess the efficacy and legality of Canada’s national security activities,” wrote former Progressive Conservative prime minister Joe Clark and former Liberal prime ministers Jean Chretien, Paul Martin and John Turner, in a letter published Thursday.

“This poses serious problems for public safety and for human rights,” they added.

Bill C-51, the Anti-terrorism Act, would provide police and the Canadian Security Intelligence Service (CSIS) with expanded powers to combat terrorism.

Jason Kenney, Minister of Defence, touted the law as necessary in a speech Thursday, in which he also warned of a “high probability of jihadist attacks from within.”

The spy agency’s work is currently reviewed by a group called the Security Intelligence Review Committee (SIRC), which tables an annual report to parliament.

The four former prime ministers warn that, when no one is watching over the agencies as decisions are made, “serious human rights abuses can occur in the name of maintaining national security.”

They said a “detailed blueprint” was set out almost a decade ago in the Maher Arar inquiry, which partly blamed CSIS for Arar’s torture in Syria.

Joe Clark later said an oversight body would act like a conscience.

“The simple knowledge that you are responsible to someone, that the things you do are going to be known, causes everybody to act more responsibly,” he said.

Liberal Leader Justin Trudeau agreed that the “simple review isn’t enough,” although he said his party would vote in favour of Bill C-51.

NDP Leader Thomas Mulcair is against the law, which he said would give CSIS powers to review environmental groups and First Nations.

Stephen Harper, meanwhile, defended SIRC on Thursday from Surrey, B.C. “We as a government are not interested in politicians doing the oversight,” he said.

A recent Angus Reid Institute poll of 1,509 Canadians found that 82 per cent support the draft Anti-terrorism Act.

But the same survey found 69 per cent agreed that “there should be additional oversight to ensure the police agencies do not go overboard with these new powers.”

The letter was also signed by five former Supreme Court justices, seven former Liberal solicitors general and ministers of justice, three past members of the intelligence review committee, two former privacy commissioners and a retired RCMP watchdog.

With a report from CTV’s Laurie Graham and files from The Canadian Press