ざるを得ない|ざるをえない

1. 1Having no choice but to do something because external circumstances make it unavoidable.Ex. 1
2. 2Having to do something against one's wishes because of an obligation or someone else's authority.Ex. 2

A somewhat stiff expression that is common in writing, news, and formal speech, though it also appears in serious conversation. It stresses that the action is reluctant and forced by the situation, which distinguishes it from neutral obligation with なければならない. It is typically used when explaining an unwelcome decision, such as a cancellation or compliance with an order.

• わけにはいかないN3わけにはいかない attaches to the dictionary form and means 'cannot do'; only ないわけにはいかない is close to ざるを得ない. ないわけにはいかない centers on being unable not to act out of propriety or responsibility and is neutral in register, whereas ざるを得ない stresses being forced by circumstances with no other choice and is more formal.
• を余儀なくされるN1を余儀なくされる attaches to nouns and, being passive, highlights an outside force; it appears often in news reports. It is used only for compulsion against your will — never for things you're happy to do — and, unlike 認めざるを得ない, it does not directly express a judgment; the two are interchangeable only in the 'forced' sense.
• よりほかないN2よりほかない attaches to a verb's dictionary form or ない-form. When it means 'this is the only thing to do,' it is interchangeable with ざるを得ない, and it can also attach to verbs like 認める to state that the facts leave only one possible judgment; however, it is not used for things you were happy to do from the start.
• 〜しか〜ないN4〜しかない, attached to the verb dictionary form, is similar: both mean 'there is no choice but to do it' and are often interchangeable, but しかない is neutral in register and more natural in daily life, and it takes the dictionary form rather than the ない-form. 〜しか〜ない also has a separate restrictive use meaning 'only ...'.
• なければいけない / なければならないN4なければいけない／ならない means you must act as the situation, rules, or common sense require; it is neutral in register and can attach to adjectives and nouns. ざるを得ない attaches only to verbs, puts more weight on having no other choice, and has a harder tone — swapping ざるを得ない for なければいけない／ならない loses the sense of being forced.
• 〜ほかないN3〜ほかない attaches to the verb dictionary form (e.g., 帰るほかない) and focuses on the fact that 'there is no other way than this'; its sense of being forced or cornered is weaker than with ざるを得ない. When meaning 'can only do X,' the two are often interchangeable, and both are formal, written-style.

Unlike なければならない, which states neutral obligation, and ほかない / よりほかない or 〜しか〜ない, which stress that there is only one option, ざるを得ない foregrounds reluctant compulsion. It differs from わけにはいかない, which says an action is ruled out on social or moral grounds, and from the more formal passive を余儀なくされる, which describes someone being forced by outside circumstances.