The extensive selection will not be fully announced but is designed to keep players on their toes ahead of the showpiece event in Qatar, and fulfil certain administrative duties such as applying for visas.
Southgate has chosen 27-year-old Webster, who plays in a three-man defence for Brighton that is similarly to England’s setup, alongside fellow Brighton stars Lewis Dunk and Danny Welbeck
Chelsea's Ruben Loftus-Cheek, Crystal Palace's Eberechi Eze, Leicester City's James Maddison and Newcastle's Callum Wilson are also thought to be on the list.
Blues defender Trevoh Chalobah is also expected to have been included and is being encouraged by his teammates at Cobham to push for his place.
However, those around the player and club are keeping tight-lipped on his potential role at the World Cup.
Southgate will announce his 26-man squad on November 10 ahead of their first match against Iran, which kicks off on November 21.
England are largely expected to stick with a similar squad from their recent September Nations League matches against Italy and Germany.
However, the Three Lions will likely have to replace Chelsea defender Reece James who suffered a knee two weeks ago.
James is yet to rule himself out and insists he can make the squad, but manager Graham Potter has stated that he will be in a knee brace for a further two weeks.
Chelsea have sanctioned an effort to speed up his recovery with a trip to Dubai to work in warm weather ahead of the tournament with fellow injured defender Wesley Fofana.
England are hopeful that Manchester City duo Kyle Walker and Kalvin Phillips will be fit to travel to the Middle East after manager Pep Guardiola gave off positive signals about their recovery.