After working for several weeks without players from Premiership finalists Sale and Saracens, Borthwick has drafted in a host of players from the two teams for the next training camp starting on Sunday.
British & Irish Lions hooker Cowan-Dickie will not be among them, however, after it was decided he would not be able to recover from a shoulder injury in time for the tournament, that begins for England against Argentina on September 9. The Exeter forward has 41 caps but has not played any rugby since January due to ankle and shoulder issues.
No.8 Mercer, who has been ineligible for England since leaving Bath for Montpellier in 2021 when he was named the Top 14 player of the year, was included in last month's preparation squad as he is due to join Gloucester next season, but has been dropped.
Fellow France-based flanker Jack Willis, was included in the wake of winning the Top 14 title with Toulouse.
There are five uncapped players in the squad - the Premiership's top try-scorer, Cadan Murley, Premiership Breakthrough Player of the Season Tom Pearson, hooker Theo Dan, prop Val Rapava-Ruskin and No.8 Tom Willis.
Veteran half-back Danny Care is also retained.
"We're excited to be able to bring a full training squad together for the first time and focus on the next stage of our World Cup preparations," Borthwick said on Friday.
"Our preparation camps over the last three weeks have allowed those players who didn't feature in the Premiership final to start to get physically equipped for a World Cup campaign.
"We hope today's announcement is the start of something special for our supporters, who I know are eagerly anticipating the tournament as much as we are."
Regarding Cowan-Dickie, Borthwick added: "Despite all his incredible work and progress, Luke's shoulder has not improved sufficiently for him to be considered for selection for the World Cup. We wish him a swift recovery and hope to see him back playing very soon."
While not named in the 41-player squad, Ollie Chessum, Ollie Lawrence, Mako Vunipola, Billy Vunipola and Jack Walker will continue their rehabilitation and are still in contention for the final 33-player Rugby World Cup squad which will be announced in August.
England will then play Wales twice, Ireland and Fiji in warm-up games.
ENGLAND TRAINING SQUAD
Forwards: Jamie Blamire, Dan Cole, Tom Curry, Theo Dan, Alex Dombrandt, Ben Earl, Ellis Genge, Jamie George, Jonny Hill, Maro Itoje, Courtney Lawes, Lewis Ludlam, Joe Marler, George Martin, Tom Pearson, Val Rapava-Ruskin, David Ribbans, Bevan Rodd, Kyle Sinckler, Will Stuart, Sam Underhill, Jack Willis, Tom Willis.
Backs: Henry Arundell, Danny Care, Joe Cokanasiga, Elliot Daly, Owen Farrell, George Ford, Max Malins, Joe Marchant, Jonny May, Cadan Murley, Guy Porter, Henry Slade, Marcus Smith, Freddie Steward, Manu Tuilagi, Jack van Poortvliet, Anthony Watson, Ben Youngs.
Rehabilitation: Ollie Chessum, Ollie Lawrence, Mako Vunipola, Billy Vunipola, Jack Walker.
