The French expression en route (meaning "on your way to") is so widely used in English that it's acceptable to not place it in italics every time you use it. What isn't acceptable is replacing the en with on.
Provence picture from Shutterstock
I understand why this happens. Because the French pronunciation of en sounds like on in English to the undiscriminating ear, people could easily assume "on route" is the correct spelling. But a sentence like "We were on route to Provence" makes no sense with the English spelling. "We were driving on route 66" is fine, but that's not where the confusion arises. Instead, it's in news headlines like this one: "Holiday hell after luggage is stolen on route to Mexico". That is simply wrong.
The easy rule of thumb? If you're not sure how a foreign expression has been adapted into English, it's safer to avoid it. (We gave the same advice for the Latin phrase per se.) Accuracy matters.
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