His statement appeared to dash hopes the Ceta deal could be signed by EU leaders and Canada on Thursday.
"We are not in a position to sign Ceta," Mr Michel said after talks with regional leaders broke down.
It is the EU`s most ambitious free trade deal to date but Belgium needs the regions` approval to sign it.
Mr Michel said he had told European Council President Donald Tusk that Belgium could not sign Ceta.
The European Commission had set Belgium a Monday deadline to make its decision on the deal agreed with Canada in 2014, after five years of negotiations.
Demonstrations
Wallonia, a staunchly socialist region of 3.6m people, wants stronger safeguards on labour, environmental and consumer standards.
Its fears echo those of anti-globalisation activists, who say Ceta and deals like it give too much power to multinationals - power even to intimidate governments.
There have also been big demonstrations in several EU countries against Ceta and the TTIP trade talks between the EU and the US.
On Sunday the European Commission presented a new clarification to Wallonia on the mechanism for settling disputes with investors.
The rules for trade arbitration are one of the thorniest issues in the deal.
But Belgium`s RTBF news reported (in French) that the latest EU document did not satisfy the Walloon politicians.
Belgium Walloons block key EU Ceta trade deal with Canada
25 October 2016
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Belgium cannot sign a key EU trade deal with Canada, Prime Minister Charles Michel says, because of objections from one of its regions, Wallonia.
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