A contestant on the popular European singing competition Eurovision has been banned from performing in the final ... and, organizers say it's because the police are investigating him.

Joost Klein -- who's representing the Netherlands in this year's edition of the contest -- will not compete in the final because Swedish police are looking into a claim made by a female member of the production crew, the Eurovision team announced in a statement Saturday.

While the European Broadcasting Union won't say exactly what the allegations entail... they do mention it occurred after the semi-final performance on Thursday night -- adding it wouldn't be appropriate for Klein to continue while the investigation proceeds.

The news comes less than 24 hours after the EBU barred Joost from rehearsal ... a shock move that sent social media into a frenzy.

Some online speculated the alleged incident may have involved Joost and another performer ... but, the EBU says that's simply not the case.

Klein was one of 26 performers who qualified for the final, taking place in just a couple hours ... but, the show will move on with just 25 songs.

BTW ... Dutch broadcaster AVROTROS put out its own version of events -- saying Klein's repeated requests not to be filmed backstage were ignored, which they say led to a threatening movement from Joost to the camera and the subsequent ban.

This whole competition's courted controversy from the beginning ... allowing a representative from Israel, Eden Golan, to participate despite tensions surrounding the country's war in Gaza. Thousands of protesters showed up Saturday to voice their displeasure she's made it to the finals.