The intelligence agency GCHQ, home to Britain’s elite codebreakers, has apologised after it clearly spelled out a swear word on social media in an answer to a puzzle it had set on Monday morning. Its Twitter followers had been invited to complete the sequence C, U, T, S, I, U, N. The correct answer was T, the fourth letter in the name of the eight planet, Neptune. But in revealing the solution on Tuesday it took a shortcut, underlining the fourth letters of “Mercury, Venus, …, Uranus, Neptune”, with an ellipsis in place of the middle planets. The tweet was taken down after less than 20 minutes and replaced with one listing all the planets. GCHQ regularly posts puzzles online to emphasise its role as home to Britain’s leading codebreakers, best known for cracking the Enigma and other Nazi codes during the second world war. A GCHQ spokesman said: “We apologise for any offence inadvertently caused by this morning’s post. We deleted it as soon as we became aware of the issue.” One Twitter user said GCHQ had committed “a brow-raising demonstration of fallibility”, and added: “See you next Tuesday, perhaps?”