McGrath, who won the Ayr showpiece on Ryalux in 2003, and Walton will face a British Horseracing Authority disciplinary panel hearing provisionally scheduled for November 11.
It is alleged the pair conspired with registered persons Mark Aspey, Carl Flint, Kristian Robson, David Stevens and David Greenwood and/or others unknown to use inside information about the likely performance of horses for betting purposes.
It is also alleged McGrath was prepared, if necessary, to omit to ride the horse on its merits so as to prevent it from winning or being placed in the race.
Aspey, Flint, Robson, Stevens and/or Greenwood are alleged, in the period between about January 1, 2011 and May 13, 2011, to have acted in breach of the rules by placing lay bets on Rumble Of Thunder when they knew they – or were reckless as to whether they – were prohibited from doing so as Listed Persons.
The BHA said a total of 57 races are potentially involved in the investigation, from October 2009 until April 2012.
McGrath’s solicitor Andrew Chalk said the rider, who is a board member at the Professional Jockeys Association, was “shocked’ at the charges and “vigorously maintains his innocence in the matter”.
Speaking on behalf of McGrath, who has not ridden this season, Chalk, partner at Withy King, said in a statement: “Richie is shocked and bitterly disappointed to be facing charges, but vigorously maintains his innocence and intends to fight to clear his name, however financially straining that may be.
“However, this case yet again highlights the vast imbalance in resources between the prosecuting authority and those who have to defend themselves, often with their livelihood at stake. A fair hearing demands a level playing field, but these cases are rarely played out on such a surface.
“Additionally, Richie has informed the Professional Jockeys Association that, whilst he fights to clear his name, he will temporarily stand down as a board member of the PJA.”
Paul Struthers, chief executive of the PJA, said: “We respect Richie’s decision to temporarily stand down from being on our board. In the meantime, it is the PJA’s role to support Richie where we are able and we will offer every assistance we can to him.”
Walton had her last runners in the 2012-13 jumps season.