The deal, known as CETA, "could be our last free trade agreement, if we are not able to convince people that we negotiate to protect their interests," Tusk said as he arrived today for a two-day EU summit to discuss trade issues.
Tusk spoke as the head of the French-speaking Belgian region of Wallonia still refused to sign off on the EU-Canada trade accord and asked that a signature next week with Canadian premier Justin Trudeau be delayed indefinitely.
Negotiations were ongoing however with Magnette mulling the latest proposal by the European Commission, the EU's executive, in his offices in Namur, 70 km southeast of Brussels.
An EU official said ambassadors from the bloc's 28 member states would meet later on Thursday to discuss the latest compromise offers.
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"We are awaiting a response by the Walloon government. The sooner would be the better," said the pro-CETA Belgian prime minister, Charles Michel.
Paul Magnette, the outspoken head of government of Wallonia, has said differences remained too wide on a contested investor protection system that has become a hot-button issue for the anti-free trade movement.
EU leaders fear that if CETA were to fail, it would send a signal to the world that it is difficult if not impossible to reach trade deals with Europe.
The struggle to close the deal is also a worry for Britain, with many seeing CETA as a potential model for EU ties with the UK after Brexit.
Activists believe both deals threaten environmental and consumer protection and offer unfair benefits to multinationals.
Tusk stressed that the problem was far bigger than just the Canada agreement, with the leaders also set to discuss tomorrow rising anger against the negative effects of global free trade that has fed the rise of populists, such as Donald Trump in the US.