Do you need a passport for a domestic flight?

[Passportto]

My passport has expired and I'm due to fly to Belfast.

I know i won't be crossing a border so presumably don't need a passport, but I'm wondering if you do need it as identification to get on a plane, regardless of where it is going?

I think a driving license would do. You need photo ID. Check the airline website.

Depends on the airline - Ryanair insist on passports for all travel

Some airlines insist on a passport - think Ryan Air do. Easyjet and flybe fine with a license or citizens card. Also expired passport sometimes acceptable as photo ID as you're not actually going through a border. Check the airlines website and it should say.

www.ryanair.com/gb/en/useful-info/help-centre/terms-and-conditions/termsandconditionsar_368204930

See DOMESTIC FLIGHT - TRAVEL DOCUMENTS ACCEPTED:
"UK: Any photo ID which matches the passenger name in the booking. Children under 16 years of age (travelling with an adult) on UK domestic flights can travel without photo identification."

Used driving licences to Belfast last year no problems.