Male MPs do not need to wear ties in the House of Commons chamber, Speaker John Bercow has said.
In a break with tradition, Mr Bercow said MPs should wear "businesslike attire" but that it was not essential for this to include ties.
Parliamentary custom is for male MPs to wear jackets and ties in the chamber.
Mr Bercow was speaking after Tory backbencher Peter Bone said he had spotted an MP - who was Lib Dem Tom Brake - asking a question tieless.
Mr Bone - who is well known for wearing rather flamboyant ties in the Commons - said he was "not really one to talk about dress sense" but asked whether the rules had changed.
"I think the general expectation is that members should dress in businesslike attire," Mr Bercow replied.
"So far as the chair is concerned... it seems to me that as long as a member arrives in the House in what might be thought to be businesslike attire, the question of whether that member is wearing a tie is not absolutely front and centre stage."
MPs should not be disrespectful towards their colleagues or the House of Commons, he said, but added: "Do I think it's essential that a member wears a tie? No."
To laughter from MPs, Mr Bercow clarified that there was "absolutely no obligation on female members not to wear ties, if they so choose".
Parliament's official rule book Erskine May only has a limited set of rules on members' dress; namely that military insignia or uniforms should not be worn in the Commons and that the custom is "for gentlemen members to wear jackets and ties".
As a parliamentary factsheet notes, the Speaker has "on a number of occasions, taken exception to informal clothing, including the non-wearing of jackets and ties by men".
In 2009, Labour MP Graham Allen was told by the former deputy speaker Sir Alan Haselhurst he was not "properly attired" when he tried to ask a question when not wearing a tie.
Two years later, Conservative MP Nadhim Zahawi apologised after a novelty tie he was wearing started playing a tune while he was making a Commons speech.