FRANKFURT (Reuters) - A German court has ruled against an online shopping site's use of Facebook's "like" button on Wednesday, dealing a further legal blow to the world's biggest social network in Germany.

The Duesseldorf district court said that retailer Peek & Cloppenburg failed to obtain proper consent before transmitting its users' computer identities to Facebook, violating Germany's data protection law and giving the retailer a commercial advantage.

The court found in favor of the North Rhine-Westphalia Consumer Association, which had complained that Peek & Cloppenburg's Fashion ID website had grabbed user data and sent it to Facebook before shoppers had decided whether to click on the "like" button or not.

"A mere link to a data protection statement at the foot of the website does not constitute an indication that data are being or are about to be processed," the court said.

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Peek & Cloppenburg faces a penalty of up to 250,000 euros ($275,400) or six months' detention for a manager.

The case comes on the heels of a January ruling by Germany's highest court against Facebook's "friend finder" feature and an announcement last week by Germany's competition regulator that it was investigating Facebook for suspected abuse of market power with regard to data protection laws.

Facebook's ability to target advertising, helped by features such as its "like" button, drove a 52 percent revenue jump in the final quarter of 2015.

Germany, Europe's biggest economy, is one of the world's strictest enforcers of data protection laws and its citizens have a high sensibility to privacy issues.

"The ruling has fundamental significance for the assessment of the legality of the 'like' function with respect to data protection," said lawyer Sebastian Meyer, who represented the consumer group in the case.

"Companies should put pressure on the social network to adapt the 'like' function to the prevailing law."

The association has also warned hotel portal HRS, Nivea maker Beiersdorf, shopping loyalty program Payback, ticketing company Eventim and fashion retailer KiK about similar use of the "like" button.

It said that four of those had since changed their practices. A first hearing in a case it has brought against Payback is due in a Munich court in May.

Peek & Cloppenburg said that it had changed its deployment of the "like" button last year and now required users to activate social media before sharing data with Facebook. It said it would wait for the court's written reasons for its judgment before deciding whether to appeal.

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A Facebook spokesman said: "This case is specific to a particular website and the way they have sought consent from their users in the past.

"The Like button, like many other features that are used to enhance websites, is an accepted, legal and important part of the Internet, and this ruling does not change that."

($1 = 0.9078 euros)

(Reporting By Eric Auchard, Harro ten Wolde and Georgina Prodhan in Frankfurt and Matthias Inverardi in Duesseldorf; Editing by Edward Taylor and David Goodman)