In reply to gammarus:
It's been discussed a number of times before... I have never tried, but it seems to boil down to: sometimes it's a problem; sometimes it's not.
My understanding is that climbing gear was previously listed as a prohibited item under 'dangerous sports equipment' but that it was subsequently removed and that current policy is that such gear *is* permitted: https://www.ukclimbing.com/forums/expedition+alpine/alpine_gear_on_the_euro...?
However they do ask that you proactively make yourself known to security during the boarding process.
But people on the forum have had trouble in the past - just search for threads with 'eurostar' in the title and you'll get examples.
I'd suggest contacting Eurostar directly and ideally getting a written clarification from them to take with you, as the last I've seen about this is a couple years old.