(Recasts with Monaco reaction)
PARIS, Feb 13 (Reuters) - Monaco said on Thursday they were "astounded" after seven French sides including Ligue 1 leaders Paris St Germain announced they will take legal action over the principality club's tax advantage.
Last month, Monaco revealed they would pay 50 million euros ($67.9 million) to the French League (LFP) as part of a deal that would "guarantee the club their participation in the championship while keeping their headquarters in the principality".
The club, owned by Russian billionaire Dmitry Rybolovlev, threatened legal action in May against a league edict that they had to move their headquarters to France, thereby ending tax exemption for their players.
Second-placed Monaco, who are five points behind PSG, have attracted several leading players over the years helped by the fact that, under the principality's laws, foreigners do not pay tax on their wages.
Arguing that last month's deal had been "rushed and non- transparent" and did not "respect some basic law principles", seven clubs said on Thursday they would bring the case to court.
"Girondins Bordeaux, (Ligue 2) Caen, Lille, Lorient, Olympique Marseille, Montpellier and Paris St Germain ... have decided to take a common legal action to get the transaction cancelled," they said in a statement.
Girondins Bordeaux president Jean-Louis Triaud told Reuters they would appeal to the highest administrative jurisdiction.
"It's a matter of public law so we're going to the administrative court, the State Council," he said.
The action has come as a surprise to Monaco, who pointed out the deal had been approved by the LFP board.
"Democracy has always been the guide rule in French football. The talks with the LFP took place within a legal framework and the deal was voted in by a large majority," the club was quoted as saying on the website of sports daily L'Equipe (www.lequipe.fr).
"All we can say is that we are astounded that it is challenged a few days later," they added.
The LFP declined to make any comment. ($1 = 0.7359 euros) (Reporting by Gregory Blachier; Additional reporting by Claude Canellas in Bordeaux; Editing by Ed Osmond)