"Spelt" vs. "spelled"

According to WordReference.com and the Concise oxford English Dictionary, both forms are correct.

spell: verb (past and past participle spelled or chiefly British spelt)

h2g2 Writing Guidelines say

Please use the word spelled rather than spelt. There was a long-running discussion about it and 'spelled' won 10 votes to 8. Besides, loads of people thought that 'spelt' was a type of wheat. Schpelled and spellted were among the other suggestions. But no, you can't use them

Mainly a correct British variant. In perhaps the best AmE dictionary:

spell

verb

spelled ˈspeld, ˈspelt (audio pronunciation) or chiefly British spelt ˈspelt (audio pronunciation) ; spelled or chiefly British spelt; spell·ing; spells

3 a : to name in order the letters of

Merriam Webster Unabridged Dictionary

Spelled vs spelt:

• In American English, spelt primarily refers to the hardy wheat grown mostly in Europe, and the verb spell makes spelled in the past tense and as a past participle.

• In all other main varieties of English, spelt and spelled both work as the past tense and past participle of spell, at least where spell means to form words letter by letter or (with out) to make clear. Outside the U.S., the two forms are interchangeable in these uses, and both are common.

• But when spell carries the sense to temporarily relieve (someone) from work, spelled is the preferred form throughout the English-speaking world. This is a minor point, though, as this sense of spell is rarely used outside the U.S., where it is most common.

• Spelled is not a recent Americanism as many people assume (including some who have commented on this post). Both spelled and spelt are old, and examples of each are easily found in historical Google Books searches covering the 17th and 18th centuries. It is true, however, that spelt was ascendant everywhere through most of the 19th century. This ended when Americans permanently settled on spelled around 1900.

(grammarist.com)

I believe the Guardian and Observer Style Guide has a better answer than just “it's different between US / UK / AUS”:

spelled is the past tense

“she spelled it out for him”

spelt is the past participle

“the word is spelt like this”

This verb has both an irregular and regular form. You can use both and both are correct. Speakers in North America and Canada use spelled while the rest of the English-speaking world seems to prefer spelt but accept spelled as well. But which is the most used one, spelt or spelled? Let´s look at Google Ngram Viewer tool which displays a graph showing how phrases have occurred in a corpus of books over the years. Spelled is more used than Spilt from 1890.

Source: https://en.learniv.com/info/en/irregular-verbs/spelled-or-spelt-what-is-correct-and-how-to-use-it/