The phrase prophet of doom designates: a person who predicts disaster, a doomsayer; also: a person who is (especially unduly) pessimistic about the future.

In Britain, this phrase has especially been used in relation to Brexit. For example, on Friday 23^rd June 2023, the British politician, author and former journalist Boris Johnson (born 1964) chose to mark the anniversary of the Brexit vote by posting the following message on Twitter:
—Note: Contrary to Boris Johnson’s claim, being in the European Union never stopped the United Kingdom having an independent foreign policy:

We have been able to take a stronger line in foreign policy—from AUKUS  to Ukraine. We chose a bold new destination for our country and it has paid off and will pay off hugely as time goes on. Don’t listen to the prophets of doom.

The earliest occurrences of the phrase prophet of doom that I have found are as follows, in chronological order:

1-: From The Complaint, a poem by ‘Mormo’, written for, and published in, The Cambridge Chronicle and Journal (Cambridge, Cambridgeshire, England) of Saturday 11^th February 1809 [No. 2,416, page 4, column 1]:

Aloft in the murky sky, o’er my devoted head,
Screams through the air the wing’d prophet of doom,
And flaps her dank pinions through heav’n’s blue vault, and bids
Love’s hapless victim prepare for the tomb.

2-: From The Hull Packet (Hull, Yorkshire, England) of Friday 27^th December 1833 [No. 2,562, page 2, column 7]:

The investigation having turned out, as we have stated, so honourable to the parties implicated, and so little to the credit of the inculpators of the corporation, we were somewhat curious to know what remarks our two contemporaries, in the plenitude of their wisdom, might think fit to favour their readers with, upon the subject. One of them, it will be recollected, when imperatively called upon by a portion of its long list of proprietors, to “speak out,” earned for itself ridicule immeasurable, by announcing the “doom” of the corporation, accompanied by certain strictures, in the worst style of the meanest of the slanderers of that body, imputing to them “gormandizing, guttling, and guzzling” (a contemptible and absurd accusation, by the bye, which it was never attempted to bring forward at all during the enquiry.) To the pages of this very disinterested adversary we looked, in the first instance, expecting to find an attempt to prove that some of the dirt they had aided in slinging upon “the corporators,” had stuck. No such article, however, was to be met with in the publication. All we could find editorially put forth on the subject of the corporation enquiry, was couched in two lines, squeezed into a notice to correspondents, to the following effect, “The present publication brings ‘the last speech and dying declaration of the Hull Corporation to a close.’” Oh! “most lame and impotent conclusion.” And has the prophet of doom no more to say upon the subject than this! The “dying declaration” of the corporation, if it is to die, the worthy of the Advertiser will observe, is no confession of guilt; and if they are to be “hanged, drawn and quartered,” they perish unconvicted, and condemned only in the opinion of those who have made up their minds upon a villainous “foregone conclusion.”

3-: From the Eastern Argus (Portland, Maine, USA) of Saturday 14^th May 1836 [Vol. 2, No. 115, page 2, column 4]:

The Gazette sets up the old story which we have heard on an average about once a year, for the last ten years, that the “Argus is going down” &c. &c. A whole brood of little Federal prints have during that space of time, started up and predicted the immediate downfall of the Argus! Still we live, and have seen these prophets of our doom, heaving their last gasp, and quietly entombed to give place to another generation of Federal Presses destined to reiterate the same prediction, and die with this idle catch word upon their lips.

4-: From The Morning Register (Dublin, County Dublin, Ireland) of Tuesday 14^th March 1837 [Vol. 13, page 2, column 1]:

The Mailman is a clever fellow, as the thinking portion of the public well know. Nevertheless he plays but second fiddle to his London correspondent. In fact he drinks his inspiration out of the charmed cup the latter provides. And what says this correspondent—for we like to go to the fountain-head? Hear it all Ireland—and Irishmen be wise in time—trimmers, this is your hour for turning your cameleon [sic] quality to account. Speaking of the English church-rates bill, this prophet of doom declares that the Dissenters are by no means satisfied with the ministerial measure—notwithstanding the vote of thanks come to by those who are privileged to speak upon their behalf; and the intelligence of our contemporary is or ought to be right, for if any party have reason to be dissatisfied the Dissenting body and not the Churchmen have fairly the grumbler’s privilege.