MPs have been told to smarten up their clothing when they return from summer recess to the Commons next week.
With the age of the virtual Commons now over, Speaker Sir Lindsay Hoyle has updated the "Rules of behaviour and courtesies" in the House.
He has alerted MPs to required dress standards, insisting clothing such as jeans and chinos are not allowed.
It represents a toughening of the advice from his predecessor, who said there was "no exact dress code".
John Bercow's last set of rules said typical business clothing was merely a suggestion.
The crackdown follows a number of moments when parliamentary fashion made headlines.
Former Foreign Secretary Jeremy Hunt was rebuked by Sir Lindsay in December for not dressing smartly enough during a Commons debate on Covid-19.
And prior to the pandemic, the then Labour MP Tracy Brabin - who is now Mayor of West Yorkshire - attracted attention when her dress slipped off her shoulder at the dispatch box.
'A privilege to serve'
Sir Lindsay's new guide states MPs should remember "the way in which you dress should demonstrate respect for your constituents, for the House and for the institution of Parliament in the life of the nation".
"Members are expected to wear business attire in and around the Chamber," it says.
"Jeans, chinos, sportswear or any other casual trousers are not appropriate. T-shirts and sleeveless tops are not business attire.
"Smart/business shoes are expected to be worn. Casual shoes and trainers are not appropriate. Men are encouraged to wear a tie, and jackets must be worn.
"It is a privilege to serve as a Member of Parliament and your dress, language and conduct should reflect this."
Sir Lindsay has also moved to clamp down on rowdiness in the Commons.
Singing is to be banned, potentially averting a repeat of the scenes from Prime Minister Boris Johnson's proroguing of Parliament in September, 2019, when Labour MPs protested by singing songs including the Red Flag and Scots Wha Hae.
The new rules state that "singing and chanting are not permitted in the chamber" and that "clapping is also not allowed as it eats into the time available for debate".
MPs have also been told to pay attention: "When listening to a debate you should not read books or newspapers or obviously devote yourself to your phone or other electronic device."
Last month, MPs packed the Commons for the first time in person in more than a year after Parliament was recalled to debate the situation in Afghanistan.
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