Wasps number eight Nathan Hughes has been banned for six weeks and will miss three of England's four autumn Tests.
Hughes' disciplinary hearing - following his citing for punching during a Gallagher Premiership game against Gloucester on 6 October - was adjourned after the forward tweeted during the initial sitting.
Hughes, who didn't feature in Wasps European defeat to Leinster at the weekend, tweeted "what a joke" on 10 October before swiftly deleting the post.
He appeared to delete his Twitter account during Wednesday evening's sitting, where the verdict was passed. A two-week sanction for the tweet will run concurrently with the suspension for striking an opponent.
The judgment was made public at in the early hours of Thursday morning – shortly before England head coach Eddie Jones was due to name his squad for the November Tests.
England take on South Africa on 3 November, New Zealand seven days later, Japan on 17 November - all of which Hughes will miss - and Australia 24 November.
The RFU statement issued was more than 1,000 words long, with Gareth Graham, the chair of the independent disciplinary panel, outlining the case in detail.
Hughes had been cited for punching Lewis Ludlow in the head while being pinned to the floor by the Gloucester flanker in the final minute of Wasps' 35-21 defeat at the Ricoh Arena.
Blows to the head are automatically considered to be a mid-range offence.
At the original hearing, it was determined that Hughes would serve a four-week suspension for the striking offence, the penalty reduced for a show of mitigation.
But on leaving the room where the hearing took place, Hughes took to Twitter. He was subsequently recalled to the room after the panel was notified of the tweet, although his legal representative had left and so the hearing was postponed.
A media release explained the postponement was "in order to deal with an issue that arose during the evening".
The reconvened hearing on 17 October heard that Hughes had only entered a guilty plea on October 10 in a bid for a reduced sanction, which had been achieved. However, that mitigation was subsequently withdrawn by the panel.
Graham said: "Where the player had previously been given credit for a guilty plea, remorse and good conduct, the panel found that that mitigation had been given on a false premise.
"He did not genuinely believe that he was guilty of the offence in question which ultimately led to his frustration as expressed in the tweet."
The tweet resulted in an additional charge of "conduct prejudicial to the interests of the union and/or the game", with Hughes pleading guilty.
A two-week suspension was applied, "to run concurrent to the six-week sanction for the punching of an opponent," Graham said.
With Saracens' Billy Vunipola and Sam Simmonds of Exeter injured, Hughes was almost certain to be chosen as England's starting number eight. Bath's Zach Mercer could now get the nod.
Whether Jones considers Hughes for the Australia Test on November 24 will be among the talking points on Thursday, as Hughes will be short of match action.
Hughes will also miss Wasps' Heineken Champions Cup clash with Bath on Saturday and the Premiership fixture with Gloucester the following week.