Canadian Prime Minister Brian Mulroney has shuffled more than half his Cabinet ministers in a bid to boost his sagging popularity and give fresh impetus to efforts aimed at averting Quebec's separation.

In an overhaul of his Cabinet on Sunday, Mul-roney declared that national unity was the Conservative government's key goal and that ministers would strive toward meeting his 1984 electoral promise for "reconciliation."Mulroney created a Cabinet "unity" committee and appointed former Prime Minister Joe Clark as referee in constitutional squabbling over the future of French-speaking Quebec.

Clark, external affairs minister for seven years, was named Mulroney's chief lieutenant on the Quebec issue. He must sell any new constitutional proposals to those provinces that only last summer scuttled an accord that would have made Quebec a willing partner within Canada.

Quebec will hold a referendum before the end of 1992 on its sovereignty, if not on any new set of constitutional proposals that would grant it more power.

After seven annual budgets and a hugely unpopular value-added tax on goods last January, Finance Minister Michael Wilson left his job to assume a new trade "superministry" designed to boost Canada's competitiveness abroad.

He was replaced in the finance job by Donald Mazankowski, who will keep his position as deputy prime minister but leaves his duties in agriculture behind.

Wilson leaves Mazankowski the legacy of a recession, a swollen accumulated budget debt and residual bitterness among Canadians over the 7 percent value-added tax.

Mazankowski, careful not to upset the value of Canadian securities on financial markets, said he would "stay the course" on Wilson's policies.

"When you've got sound policies, you don't have to be different," Mazankowski told reporters.

The woman representing Canada on the world stage as external affairs minister will be the tough-talking Barbara McDougall, who has handled immigration, women's issues and junior finance portfolios in the Conservative government.

Trade Minister John Crosbie, who signed the free-trade deal with the United States and was preparing for talks on extending that to Mexico, left his office to take over in the fisheries department.