Grewcock remains suspended from August 4 to September 15 - a period which covers 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.
The Bath veteran was originally banned after being found guilty of punching opposite number Thibault Privat in the European Challenge Cup final defeat to Clermont Auvergne.
As a result, he was omitted from England's 47-man World Cup training squad and head coach Brian Ashton indicated Grewcock's only hope of playing at the tournament was in mounting a successful appeal.
But the independent panel, which convened in Birmingham on Tuesday night, found "there had been no error on the part of the original committee's decision".
Grewcock was ordered to bear the costs of the appeal hearing and the ERC's legal costs.
Bath issued a brief statement which read: "Bath Rugby would like to thank the ERC for convening an appeal hearing promptly.
"The club is disappointed with the outcome but accepts the decision of the appeal committee."
Ashton indicated he could not carry a suspended Grewcock in his 30-man World Cup squad.
Not only would he miss the entire preparation and the key pool game against South Africa, but Grewcock would not have played any rugby for four months by the time England face Samoa in Nantes on September 22.
Grewcock is typically England's second row enforcer but Ashton's has other options, including Martin Corry who he likes to employ at lock.
Ben Kay, Tom Palmer, Steve Borthwick, Simon Shaw and Louis Deacon are the specialist locks Ashton named in the 47-man training currently being put through their paces with the Royal Marines.
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 the All Blacks in 1998.
He 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.