Is there a word for a "promise breaker"?

I'm somewhat vexed in that I cannot think of a word that means a "promise breaker" or "person who breaks a promise". There are words that may subsume that, such as "miscreant" or "liar", but I cannot come up with an English word that is limited to a person who breaks promises.

The closest I've come is "piker", which is (a) informal and (b) limited to Australia/NZ, but means (according to the dictionary on my Mac):

a person who withdraws from a commitment.

Is there a more formal and common word that can be used to refer to people who break promises?

Solution 1:

Reneger:

renege, renegue vb (intr; often foll by on)
to go back (on one's promise, etc.)
reneger , reneguer n

Reneger vs. oath breaker ngram:

Solution 2:

A personal favorite slang term for an unreliable person is a flake. A flake says they are going to do something and then they don't follow through. The definition is not limited to promise breaking, however.

Solution 3:

There's "oathbreaker". It isn't very common these days though.

Solution 4:

Warlock - if you're into really Old English:

Middle English warloghe, from Old English wrloga, oath-breaker : wr, pledge; see wr-o- in Indo-European roots + -loga, liar (from logan, to lie; see leugh- in Indo-European roots).

(Note: the link originally went to Wikipedia's Warlock, which had a brief description, including roughly the oathbreaker meaning. Apparently the word has changed, to a D&D character...)