Speaker John Bercow - often heard calling for "Order!" among the rowdy MPs - said he did not think it essential for them to don a tie.
"It seems to me that as long as a member arrives in the House in what might be thought to be business-like attire, the question of whether that member is wearing a tie is not absolutely front and centre stage," he said, himself wearing a patterned purple tie.
The fashion revolution was started by Liberal Democrat lawmaker Tom Brake who dared address the House of Commons without a tie, prompting MP Peter Bone from the ruling Conservative Party to ask whether the rules had changed.
While there is no obligatory parliamentary dress code, a jacket and tie is customary for men while women are expected to dress equally formally.
"Who says Brexit dominates the political agenda? My attempt to ask a PMQ tieless has roused traditionalists from their slumber," Brake wrote on Twitter following Bercow's intervention.
But Conservative lawmaker Geoffrey Cox appeared less pleased, tweeting: "Shocking. Tieless. Where will it all end?"
The new move was also applied to political reporters, who have been refused entry when tie-less in the past, to mixed response.
But The Sun's Matt Dathan praised parliament for "finally moving out of the 19th century", while Buzzfeed's Alex Spence delighted in taking his tie off and being given a thumbs-up by the doorman.
The overhaul is the latest move by Bercow to bring parliament into the modern day, having dropped the requirement for clerks to wear wigs earlier this year.
Despite relaxing the rules, MPs and politics enthusiasts alike can still purchase a tie from the official Houses of Parliament shop.
Lawmakers have in the past been chided for not wearing a tie in parliament, including a 2009 clash between the deputy speaker Alan Haselhurst and MP Graham Allen.
"It is not appropriate to address the House without being properly attired," Haselhurst told the lawmaker, who later slammed "Neanderthal MPs" for blocking a youth debate in the Commons, according to The Telegraph.