IT'S US WHO + singular or plural?

Could anybody help me clear up this nagging doubt:
do you say "it's us who have to apologize" or "it's us who has to apologize" ?
In the singular I know I would say "It's I who have to apologize" and "It's me who has to apologize" but what about the plural pronouns?
Thank you guys for your valued opinion
Waugh

Thank you for your quick response.
However, "It's me who has to apologize" must be correct as I read it not ten days ago in a British magazine. If you say "It's I who have to apologize" similarly you should say "it's we who have to apologize", shouldn't you? The plural after "us" would be natural, then why did Nick Hornby, a famous English novelist, use the third person singular in his novel "How To Be Good"? That's what set me thinking. Anyhow, I'll await further comments.
Thanks a million buggah.

And who says all British magazines speak English correctly?

My way of thinking about this one would be that we use the nominative case ('I', 'he', etc.) when the subordinate clause refers to its subject (sorry, I don't know how to say this in grammatically correct terms), as in 'It is I who have to apologize', and we use the causative case ('me', 'him', etc.) when the subordinate clause refers to its object, as in 'It is her you should take with you.'

Collins English Dictionary says:
Although the nominative case is traditionally required after the verb 'to be', even careful speakers say 'it is me' (or him, her, etc.) rather than 'it is I' in informal contexts.

It's we who have to apologize.