noun
informed sources; well-informed circles
Used in news and formal contexts to refer to people or groups with inside information, often cited anonymously.
消息筋によると、首相は来月辞任する意向だ。
According to informed sources, the prime minister intends to resign next month.
関係者 is a broader term for people involved or concerned, while 消息筋 specifically implies those with inside information, often used in journalism.
Compound of 消息 (news, information) and 筋 (line, channel, source). The exact historical derivation is uncertain, but the combination naturally yields the meaning 'information channel' or 'informed source'.