Despite opposing Scottish independence, the Prime Minister let his 19 European MPs strike a deal in Brussels with the New Flemish Alliance, or Nieuw-Vlaamse Alliantie (N-VA).
The N-VA's four MEPs joined the European Conservatives and Reformists (ECR) group in the European Parliament last week.
The third largest bloc in the parliament, the ECR was founded in 2009 by the UK Conservatives, and 19 of its 68 members are UK Conservatives.
One of the N-VA's keenest supporters in the UK is the Tory mayor of London Boris Johnson.
In a recent video message of support, now on YouTube, he says one of his daughters was born a Belgian and he lived in the country many years.
"So I think I can say something about the very interesting points of comparison between what we British Conservatives are trying to do and what the N-VA is doing in Belgium under [leader and mayor of Antwerp] Bart De Wever.
"Obviously we're both trying to cut crime, to cut taxes, and to get people into jobs.
"That's what the N-VA is all about. That's what Conservatives are all about too.
"Go Antwerp! Go Brussels! Good luck to the N-VA! Good luck to Bart De Wever!
"Boris Johnson, over and out."
However, Johnson failed to mention the N-VA's driving aim is to break up Belgium and for the northern Flanders area to become a new EU state.
Its website states: "The N-VA doesn't want a revolution and it is not looking to secede from Belgium. We are, however, striving for more democratic and more efficient structures.
"We want to take this step by step.
"Our final target is indeed an independent Flanders as a European Member State, but the progression to reach it is gradual and must occur in a democratic manner."
Formed in 2001, the centre-right party is economically liberal, big on green issues, and advocates sole use of the Dutch language in Flanders, including the compulsory teaching of Dutch to immigrants.
An SNP spokesman said: "This just points up the hypocrisy of the Tories.
"While they oppose independence for Scotland, they are in formal alliance in Europe with a Flemish independence they praise to the skies.
"The Tories want a No vote because keeping Westminster is the only way they get to govern Scotland with just one MP here."
A Scottish Conservative spokesman said: "We're in the ECR as fellow economically liberal parties who oppose EU federalism, not because of any agreement on internal politics within an EU Member State."