What's this caterpillar?

What's this caterpillar?

Observation - What's this caterpillar?. Description: I haven't found this large caterpillar in guides to UK / European butterflies & moths. If anyone knows what it is, please tell me & I'll update the identification. Thanks.

I haven't found this large caterpillar in guides to UK / European butterflies & moths. If anyone knows what it is, please tell me & I'll update the identification. Thanks.