The actual sentence, or if it reveals too much about your client, a summary of the background, would help a lot, as there are several ways to translate this.

connected with labour law

(or simply labour law used adjectivally)
following labour law principles/procedures

industrial procedures
is another suggestion in the dictionary.
(Gyldendal online)

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Note added at 43 mins (2013-12-16 15:30:43 GMT)
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... has already lost two cases in industrial procedings, where errors were found in wage payments toworkers on the Metro

Something likethat as a first suggestion - you may want to re-word it.

det fagretslige system refers to the Danish labour market system, and the rules therein, which employers and employees are subject to. I'm not certain if this system includes/excludes labour unions - do any Danes know?

"fagretslig el. fagretlig - som drejer sig om forholdet mellem arbejdsgiver og arbejdstager"
see Politikens NUDANSK ORDBOG, JP/Politikens Forlagshus A/S, DK-1456 København K, 2010

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Note added at 16 hrs (2013-12-17 06:50:56 GMT)
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As I understand, this expression is not about the law, but a system with an accent on employer-employee relationship. In this connection, please see 'Ordforklaring - fagretsligt system' .
https://www.lfs.dk/640

I think this is clearly "pertaining to labour legislation" - and that is how it should be expressed.

(though if you had more context, minor changes might be in order).