The Toronto lawyer who successfully challenged the previous Conservative government over one of its Supreme Court judge nominees is setting his sights on a new target: Canada's free trade deal with the European Union.

Rocco Galati has filed a statement of claim in Federal Court arguing that the Comprehensive Economic and Trade Agreement, as the deal is known, is unconstitutional.

The complaint focuses on controversial provisions in the Canada-EU deal, similar to those contained in the North American Free Trade Agreement, that would let companies sue the government under certain circumstances.

The statement of claim also takes issue with the federal government's tradition of signing and ratifying free trade deals and other international agreements without prior approval from Parliament or the provinces.

"Once it's signed and ratified, we're bound," Galati said in an interview Monday.

"That's the problem. But other trade partners like the U.S. and Europe, they don't get bound this way. They have to put it through their parliaments and legislative houses before they can sign and ratify."

Galati is developing a reputation for taking on Ottawa. The Supreme Court of Canada sided with him three years ago when he challenged the Harper government's attempt to appoint Justice Marc Nadon to the top court.

The court challenge, filed Friday on behalf former Liberal cabinet minister Paul Hellyer and two other people, comes with the pact between Canada and the EU on life support thanks to a small region of Belgium standing firm in opposition to it.

Lawmakers in the region known as Wallonia have refused to approve the deal, which has been seven years in the making. Among their concerns is the investor-state dispute settlement mechanism, one of the subjects of Galati's court challenge.

"It's ironic that everybody is dumping on the Walloons," Galati said. "They have a very similar constitution to ours except they're respecting theirs. So I don't know why they're being criticized for respecting their constitution."

Asked whether the agreement meets the requirements of Canada's Constitution, International Trade Minister Chrystia Freeland replied, "Absolutely." Her office refused to comment further.

The statement of claim gives the federal government 30 days to respond.

'Royal prerogative'

University of Ottawa constitutional law expert Errol Mendes said it's been long established that the Constitution gives the federal government the power to sign and ratify treaties, just as it can deploy troops overseas without parliamentary approval.

"(Galati) may have won on Nadon, but I think he's going to lose on this," Mendes said. "It's basic 101 constitutional law that the federal government has the right to sign and ratify international treaties under the royal prerogative."

Galati isn't the first to turn to the courts to stop the trade deal. Several groups in Germany raised concerns about parts of it coming into effect once it is signed, even though the country's parliament wouldn't have voted on it yet.

Germany's constitutional court rejected the request for an emergency injunction two weeks ago, but did rule that the country must be allowed to leave unilaterally if the deal is found to be unconstitutional. The court also limited which parts of the deal could come into effect without the German parliament's approval.

Another case on the deal's constitutionality is still before the German courts.