BRUSSELS -  Belgium on Saturday officially signed the landmark EU-Canada trade accord after a drama that saw Belgian regions threaten to torpedo years of negotiations.

"Nothing is simple in Belgium but few things are impossible," tweeted Belgian Foreign Minister Didier Reynders after signing the pact on behalf of his country.

EU Trade Commissioner Cecilia Malstrom, who negotiated the deal, attended the signing with Belgium the last of the 28 EU countries to approve the Comprehensive Economic and Trade Agreement (CETA).

On Sunday, the European Union and Canada will formally sign the trade accord at a summit in Brussels.

Canadian Prime Minister Justin Trudeau hailed the deal as "a good sign in an uncertain world" in a phone call with EU president Donald Tusk on Friday, an EU source said.

CETA will remove 99 percent of customs duties between the two sides, linking the single EU market of 500 million people with the world's 10th largest economy.

Sunday's summit will begin at 0930 GMT, with the agreement due to be signed at 1100 GMT, Tusk's spokesman Preben Aamann said on Twitter.

Just days ago, CETA -- the most ambitious deal ever negotiated by the EU -- had been left hanging by a thread due to protests from Belgium's Wallonia region and other French-speaking communities over its potentially harmful effect on local interests.

Trudeau had been due to arrive in Brussels on Thursday to sign the deal but his trip had to be called off, with leaders including Tusk warning that the debacle was further damaging EU credibility following Britain's shock vote to leave the bloc.

After hitting deadlock in talks with Walloon leaders last week, an emotional Canadian Trade Minister Chrystia Freeland dismissed Brussels as "incapable" of achieving an international agreement.

Paul Magnette, head of the southern, French-speaking Wallonia region, had fought for regional farming interests and guarantees against international investors forcing governments to change laws against the wishes of the people.

Belgian Prime Minister Charles Michel has said the fraught talks with Wallonia that were finally resolved on Thursday "did not change a comma" in the deal, but Magnette says he received assurances from the federal government of strengthened social and environmental protection.

"Comprehensive Economic and Trade Agreement amended, CETA corrected -- that is fairer than the former CETA and offers more guarantees," Magnette said.

The EU was cleared to sign the pact shortly after Wallonia's parliament voted to approve the deal, along with that of the Brussels-Capital region and the Federation Wallonia-Brussels.

Once signed by the EU and Canada on Sunday, the trade pact will go into effect on a provisional basis, pending full member state ratification -- a process that could take years.

Prime Minister Robert Fico of Slovakia, which currently holds the EU presidency, hailed the delayed approval of the deal as "a milestone in the EU's trade policy".

"It has the potential to set the way forward for future trade deals," he said.

Hinging on CETA's outcome are complex EU trade negotiations with other countries, including an even bigger and more controversial proposed deal with the United States known as the Transatlantic Trade and Investment Partnership (TTIP).

Wallonia has already warned it would never accept a trade deal that does not have same guarantees as those in the new CETA.

"From now on, we can draw the lesson: with CETA improved, TTIP is dead and buried", Magnette said.