The No 8 will miss the first three autumn internationals, deepening Eddie Jones' selection headache.
Hughes was initially banned for four weeks for punching an opponent.
But he tweeted "what a joke" after leaving the hearing last week, forcing the panel to reconvene yesterday.
And in a 1am announcement this morning, it was decided to reinstate the six-week suspension, which will run concurrently alongside a two-week one for the tweet.
The RFU said in a statement: "The Player is not, at the time of the reconvened hearing, named in the England squad and therefore his sanction of 6 weeks will apply to the next Wasps fixtures.
"Whilst the Panel accept that the tweet was written in a moment of frustration and was deleted quickly by the Player, it had already been picked up on social media and was then discussed publicly.
"Taking into account the proportionality of the total sanction, this period of 2 weeks is to run concurrent to the 6 week sanction for the punching of an opponent.
"The Panel make it clear that tweets and comments such as this undermine the Game, the core value of respect and the disciplinary process.
"The Player is free to play again on the 20 November 2018."
With Billy Vunipola also injured, England boss Jones is without his two first-choice No 8s.
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