The Bath lock, 34, was found guilty of punching opposite number Thibault Privat in last month’s European Challenge Cup final defeat to Clermont Auvergne.
The suspension runs from August 4 and rules Grewcock out of England’s three World Cup warm-up Tests and the first two matches of the tournament – including the vital clash with South Africa on September 14.
England head coach Brian Ashton is set to name his World Cup training party on Friday and must decide whether it is worth carrying Grewcock in the final 30-man squad.
It seems highly unlikely. By the time England face Samoa in Nantes on September 22, Grewcock would not have played any rugby for four months.
At the age of 34, the ban could also spell the end of Grewcock’s 69-cap England career.
Grewcock’s only hope could lie in an appeal, which has to be made within 72 hours. Bath plan to wait until they receive the full disciplinary report before making a decision.
If Grewcock ends up missing the tournament on disciplinary grounds, it would perhaps sum up a chequered career for a player from the no-holds-barred school of rugby.
Grewcock became only the second player to be sent off for England when he was given his marching orders for illegal use of the boot against New Zealand on the 1998 tour.
Grewcock received a five-week ban on that occasion and picked up suspensions on his two subsequent trips to New Zealand.
In 2004 Grewcock was banned for six weeks after being cited for stamping against the All Blacks. And the following year he was suspended for two months after being found to have bitten Keven Mealamu in the first Lions Test.
Grewcock has been sin-binned three times in the last three years for England while, at club level, he has one of the worst disciplinary records in the Guinness Premiership.
He has been sent off twice playing for Bath. The first red card was quashed on appeal but the second, for punching Lawrence Dallaglio in the 2003 Parker Pen final, cost him a place on England’s summer tour.
Grewcock earned a place in England’s 2003 World Cup squad but missed the triumph in Australia after breaking his hand in the pool win over Uruguay.
Now he appears set to miss out on England’s title defence.