The ban 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.
Ashton, who is set to name his World Cup training party on Friday, must decide whether he can afford to carry Grewcock in the final 30-man squad.
It seems 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 conceivably spell the end of Grewcock's 69-cap England career with the likes of Tom Palmer, Louis Deacon, Dean Schofield and Chris Jones all growing in experience.
They are all likely to join Ben Kay, Steve Borthwick and possibly Simon Shaw in Ashton's training squad this Friday.
While Ashton considered his selection options, Bath were last night waiting to receive the full disciplinary report before making a decision on whether to lodge an appeal.
If Grewcock ends up missing the tournament on disciplinary grounds, it would sum up his chequered career.
He 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 in 1998.
He received a five-week ban on that occasion and incurred suspensions on his two subsequent trips to New Zealand.
In 2004 he was banned for six weeks after being cited for stamping and the following year he was suspended for two months after being found to have bitten Keven Mealamu in the first Lions Test.
At club level, Grewcock has one of the worst disciplinary records in the Guinness Premiership and 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.