What is the origin of the "once upon a time" idiom as the way to begin a fairy tale?

"Once upon a time" is the traditional way to start a fairy tale using the English language. But how traditional is it? I'm trying to find the first uses of this expression with this purpose. So far I have found a text from 1672 using Ngram that says:

Could Mistriss Mopsa her self have furnished you with a more pleasant and worshipful Tale? It wants nothing of perfection, but that it doth not begin with Once upon a time?

So by the 17th century it already was a traditional expression for beginning tales. But I would like to know how far does that tradition go. What are the first English texts that reflect the use of "once upon a time" to start a tale? When were those texts written? Are there any older versions of the idiom that could be the origin of this one?

This expression and similar are found under definition 8a(g) in the MED:

in phrases freq. introducing a narrative: ones on a time, in (on, upon) a time, in (on) on time, etc., once upon a time, once; also, in adv. constructions without prep.: o (on) time, on a certain occasion, once

Coincidentally, you can also see "on a time" in this free OED page.

One of the first examples is from c1225(?c1200) from Þe Liflade of St. Juliana. Although it's only in the third paragraph, it still serves to start off the rest of the story:

Wes iþon [Roy: bi þon] time as þe redunge telleð þe modi Maximien keiser irome...

This translates as:

In that time, as the legend tells, the proud emperor Maximian was in Rome...

St. Katherine also from circa 1225 has a similar line:

Constantin ant Maxence weren on ane time [c1225 Bodl. on a time] as in keiseres stude hehest irome.

Which translates (according to me) as:

Constantine and Maxentius were on a time...

There's also Speculum Guy, c1330:

A tale i wole ȝou telle Off an eorl..Gy of Warwyk was his name, Hou on a time he stod in þouht.

A tale I wish to tell you of an earl... Guy of Warwick was his name, how on a time he stood in thought. (translation mine)

Chaucer, in his Knight's Tale, uses the wording "ones on a time", but this happens in the middle of the story.

It seems the wording with upon came later:

Galien seith and tellith þat vp-on a tyme he sey a child þat had þe fallyng evell.

Galien says and tells that upon a time he... (translation mine)
_{A Middle English Translation of Macer Floridus de Viribus Herbarum, a1450}

Also related is "once upon a day", which dates back to circa 1380:

Onys..oppon a day..he slow kynges three.
_{Sir Ferumbras}

The Wessex Gospels (1175) contain the words (in Matthew 24:44) :

mannes sune wile cumen on þare tyde þe ge nyten

TR Interlinear (Wessex at foot of page)

The son of man will come on a time (or on 'some' time) that ye reckon not.