Lesson learned or Lesson Learnt

Dear Teachers,
Greetings. I would like to get clarification in using lesson learned or lesson learnt.
I always came accross these 2 words are use interchangeable but I would really love to know which one is the most appropriate and formal usage.

Thanking you and appreciate your comments.

Welcome to the forum, Elmy.

They are both appropriate formally and informally.

'Learnt' is more commonly seen in British English and 'learned' in American English.

They are both pronounce 'lernt'.

'Dreamed' and 'dreamt' are similar, I think.

There is no "t" sound in "learned" in AmE.

SoothingDave said:

There is no "t" sound in "learned" in AmE.

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You can still hear larnt from some Appalachians, but the -t ending is certainly rare in spoken American English and, I think, practically unknown in the written language.

Sue01 said:

'Dreamed' and 'dreamt' are similar, I think.

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There's no similarity in pronunciation between those two though.

Dreamed = dreem'd
Dreamt = dremt

(Sorry, I don't do phonetics so that's my best stab at showing how they're pronounced.)

RobertJ said:

In my community it's usual plural, i.e. "lessons learned".

"We lost the election. Let's review the lessons learned."

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It can be singular, but I usually see it as a plural with the -ed ending in this sort of context in BrE. The phrase would look odd to me with the -t ending.