on route vs en route

I grew up English, in a French city. En route is the only thing that feels right when reading, but I am flexible. On route looks wrong but makes sense, so I suppose it is acceptable.

Google Books finds instances of en route in otherwise predominantly English texts dating to the late eighteenth century. The earliest is from an entry in Wolfe Tone's diary for July 31, 1796, in Memoirs of Theobald Wolfe Tone: Written by Himself (1827):

Received my pay, "and are all a drunk as so many swabbers." I insist upon it this is a very good quotation, from Rigdum Funnidos. The monotony of my life just now will appear from the stupidity of these memorandums, and especially from the dulness of my jokes. I cannot express how much I long to be "en route."

Tone was in Paris at the time, so his milieu may well have influenced his word choice. But either way, it's a start. In the first half of the nineteenth century, government functionaries—military, economic, and political—frequently used en route, and this is the period when the term became naturalized in English.

Another fairly early instance, this time involving the longer phrase "en route to," appears in a journal entry for July 1821 in James Tod, "Personal Narrative," in Annals and Antiquities of Rajast'han: Or the Central and Western Rajpoot States of India (1832):

Oodipoor, July 1821.—When I concluded the narrative of my journey in October last year, I had no expectation that I should ever put my foot in the stirrup again, except en route to Bombay, in order to embark for Old England; but 'honhár!' as my Rajpoot friends exclaim, with a sigh, when an invincible destiny opposes their intentions.

The earliest definite Google Books matches for on route in the relevant sense appear in a series of letters dispatched by Charles Irby and James Mangles on December 6, 1817, and February 17, 1818, and published in Travels in Egypt and Nubia, Syria, and Asia Minor; During the Years 1817 & 1818 (1823):

Late in the evening we arrived at Yead, a village about an hour's distance from Baalbec; the horses having been on route from two o'clock in the morning, without any food, about fifteen hours, which caused us to blame our guide much, as we might have brought fodder with us from Aden, had we known how uninhabited the country was through which we had to pass.

...

The weather turned out very wet this day, and after we had been on route about three hours, being two hours' distance from Antioch, we perceived some cottages, and being thoroughly wet, through baggage and all, we requested shelter.

...

The following morning, at dawn, we were on route; we saw twenty-three white gazelles, and witnessed the removal of an Arab camp; the moveables being all placed on the camels' backs; the women with their children slung over their shoulders, and the flocks following, presented altogether an interesting sight.

A similar instance appears in "Postscript to Asiatic Intelligence," in The Asiatic Journal and Monthly Miscellany (October 1826), where the phrase "on route to" first appears:

The land column from Yandaboo comprises H.M.'s 87th regt., a detachment of the body guard and horse artillery, the 26th, 28th, 38th, and 43d regts. of native infantry and pioneers. The detachment on route to Arracan consists of the 18th N.I. with Lieuts. Trent and Bissett of the Quarter-master-general's department ; and that on route from Prome, of a detachment of horse artillery and body guard, the 32d N.I. and pioneers.

It thus appears that English writers have been using the spelling "on route" as a variant of "en route" for very nearly as long as they have been using the latter term. A Google Books Ngram chart for "en route to" (blue line) versus "on route to" (red line)—a phrase I use for comparison because it filters out a lot of false positives for "on route" of the the type "on route 66"—for the period 1800–2005 shows a fairly steady rise in the overall frequency of "en route to" over the past two centuries, along with a low-level but persistent incidence of "on route to":

A search for the same two phrases over the period 1800–1900 confirms that most of the wavelets for "on route to" depicted in the graph represent actual occurrences of the phrase and not false positives. On that record, I don't think we can say that "on route to" is a spelling error; it's simply a (relatively uncommon) variant spelling.

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Update (9/26/2020): Earlier instances in English of 'en route' and 'on route'

Hathi Trust database searches for en route and on route turn up some earlier instances of these expressions than the earliest ones I found in Google Books searches.Whereas the earliest Google Books match in an English text for en route was from 1796, Hathi Trust turned up on from 1774. From a letter dated August 4, 1774, in William Dalrymple, Travels through Spain and Portugal, in 1774; with a Short Account of the Spanish Expedition Against Algiers, in 1775 (1777):

My Dear Sir, As you are so exceedingly anxious to have the plan of the new-formed academy at this place [Avila, Spain], I shall endeavor to give you an account of it, together with the general state of the military establishment, discipline of the army, &c. As I took the Escorial en route, it will probably afford you some entertainment to have my cursory remarks on that surprising edifice. I shal therefore give you my journal from Madrid, which I left the 30th, at five o'clock in the morning.

As for the spelling on route, Hathi Trust searches turned up two examples from the late eighteenth century. From a letter from General Harrington to Major General Gates dated September 21, 1780, in The State Records of North Carolina, volume 14, 1779 '80 (1896):

I should have resigned immediately on receipt of Colo. Wade’s Letter [indicating that Harrington had lost his command to another officer], but as there was the highest probability that Major Weems, then near the mouth of Lynch’s creek, and pushed on by Doctor Mills, would visit the Cheraw District, where Mills used to reside ; and although greatly distressed by the sufferings of the remains of my Family, then on route to Virginia, I could not support the thought of abandoning the Brave, the Generous, the Distressed, in the three Peedee Regiments, to the fury of a cruel and vindictive Foe as long as there was the least probability of its being in my power to assist them ; and I flattered myself, Sir, as it was so clearly our Interest to assist those brave men in protecting their property, their Grain & their Mills especially, even for our own sakes, that I should have been honoured with Orders to have marched with the Party under my command to Peedee before our Friends there had been obliged to fly, as there was no doubt, with their assistance, of our being able to drive Mills’ Banditts, who still continue to plunder, burn & destroy all before them.

And from C. Este, "From Louvaine to Liege," in A Journey in the Year 1793, Through Flanders, Brabant, and Germany, to Switzerland (1795):

Our lively friend, we found, had been no slight traveller. And was then on route, as we understood, to mix in the hunting parties of the English Viscount F——, in the electorate of Cologne and Westphalia.

These findings don't alter the conclusion I drew at the end of my original answer—that "English writers have been using the spelling 'on route' as a variant of 'en route' for very nearly as long as they have been using the latter term," and that it seems reasonable to view "on route" as a variant spelling rather than as a spelling error—but they do push back the earliest dates that I've been able to identify for each term by 22 years (in the case of en route) and by 38 years (in the case of on route).