Regrettably means unfortunately, as one is sorry to say. Regrettably late, regrettably, we cannot, regrettably common.
It is the formal register of an apology and does not admit fault, which is why it appears in letters that decline things. It is distinct from regretfully, which means with regret felt by the speaker.
1adverbregrettablyUnfortunately, as one is sorry to say.
“Regrettably, tonight's concert has been cancelled because the singer is ill.”
regrettably is a manner.
From regrettable + -ly.
Etymology from Wiktionary, CC BY-SA