Premiership and European champions Saracens have been deducted 35 points and fined £5.36m for breaching salary cap regulations in three seasons.
The punishment comes after an investigation into business partnerships between chairman Nigel Wray and some of the club’s players.
Saracens will appeal and have described the sanctions as “heavy-handed”.
A Premiership Rugby statement said both punishments would be suspended if Saracens chose to review the decision.
Official league tables have already been updated to show Sarries bottom of the Premiership with -26 points.
Saracens ‘absolutely devastated’ by punishment
The charges relate to a failure to disclose player payments in each of the 2016-17, 2017-18 and 2018-19 seasons.
Saracens previously claimed they “readily comply” with salary cap rules and were able to spend above the £7m cap because of the high proportion – almost 60% – of home-grown players in their squad.
“This is absolutely devastating for everyone associated with this amazing group of players, staff, partners and fans,” Wray said in a statement.
“It’s been acknowledged by the panel that we never deliberately sought to mislead anyone or breach the cap.
“That’s why it feels like the rug is being completely pulled out from under our feet. We will appeal all the findings.”
During an independent disciplinary panel hearing, Saracens saw their challenge of the validity of the regulations on competition law grounds rejected.
Premiership Rugby introduced their salary cap in 1999 to ensure the financial viability of all clubs and the competition.
The regulations are also designed to control inflationary pressures on clubs’ costs and provide a level playing field for clubs and a competitive Premiership.
“The decision upholds both the principle of the salary cap and the charges brought following an extensive investigation,” a Premiership spokesperson said.
“We’re pleased this process has reached a conclusion.”
‘The biggest story in English club rugby history’
Analysis: BBC rugby union correspondent Chris Jones
Saracens have been the dominant force in the domestic game for the best part of a decade – scooping seven major titles and providing the spine of the England World Cup team – but that success will now be considered tainted.
How long has it been going on? Will the club keep their titles? What will happen with their appeal, given they insist they were involved in legitimate business dealings with players? What happens now to the current squad, which may need to be dismantled, especially with a £5m fine and the threat of relegation?
And what do players, coaches and fans at other clubs think, given everyone is affected in some way by this? On that note, do any other clubs in the league have something to hide?
Like with the Bloodgate scandal involving Harlequins 10 years ago, the fallout to this will be significant and lengthy, and will damage the integrity of the Premiership just at the point the league is looking to launch a global expansion.
This is probably the biggest story in English club rugby history.
Sarries a sporting powerhouse
Saracens have developed into a true sporting powerhouse during the past decade, winning five Premiership titles and three European Champions Cups since 2010-11.
Two of those domestic titles came in the timeframe that Premiership Rugby have been investigating, with Mark McCall’s side winning 53 of 72 league and play-off matches during that period.
They have been equally dominant in European competition, having lifted the trophy in three of the past four seasons.
In the five seasons Saracens have finished as Premiership champions, a 35-point deduction would have meant them not reaching the play-offs by finishing in the top four, but would also not have seen them relegated.
They would have finished 10th last season had the same punishment been imposed.
Saracens have won two of their three Premiership matches so far this season.
They started the current Premiership campaign with a significant number of their star players still on World Cup duty.
Of the 31-man squad representing England in Japan, seven players came from Saracens – including captain Owen Farrell and forwards Mako and Billy Vunipola.
Full-back Elliot Daly, another member of the side that went on to lose the final against South Africa on Saturday, will join Sarries following the conclusion of the tournament.