Carry digicard at all times?

Help me out here guys please.

No if, buts, maybes, rdc talk, my mate said etc etc.

I worked as a bus driver for 10+ years, digicards were not needed because the buses were not fitted with tachos.

I have been told, that if you have been issued a digicard, you MUST, by law carry it even when driving buses with no tacho fitted.
I know you must carry digicard if driving a lorry which is fitted with a paper tacho.

Ive been on google and cannot find anything say I MUST, by law carry my digicard if I drive a bus fulltime.

Anyone got a link to the law?
Without sounding funny, I mean a link to the LAW, not a website that advises that you should carry your digicard.

Cheers.

gov.uk/guidance/drivers-hou … ic-driving

Section 3.4 Records

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El Deano:
Drivers’ hours and tachographs: buses and coaches - 3. Mixed EU/AETR and GB domestic driving - Guidance - GOV.UK

Section 3.4 Records

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Thanks, so, as I read it, when driving on uk domestic rules(which most service buses do) no need to have digcard card with you.