The Three Lions boss will name his selection, which has been increased from the usual 23-man pool, on Tuesday.
Sources believe that two of the biggest names who could miss out are Liverpool's Trent Alexander-Arnold and Manchester United's Mason Greenwood.
Alexander-Arnold, although regarded as one of Europe's top full-backs, suffers from the fact that England are very well stocked at right-back.
Southgate has huge regard for Kyle Walker - who can play centrally in a three - and Kieran Trippier, and with Chelsea FC's Reece James also looking likely for selection, Alexander-Arnold is in for a nervous wait and that is signified in his odds of 4/7, which are longer than those of his rivals - with Walker at 1/16, Trippier 1/2 and James 2/5.
Indeed, four right-backs could be a push and another who could play full-back, and who could threaten Alexander-Arnold, is Arsenal's Buakyo Saka. Although they are not rivals positionally, they could be in terms of the final slot, and Saka's natural fit on the left could give him the edge. His odds are the same as the Liverpool man at 4/7 to make the squad.
United star Greenwood could miss out due to the fact he doesn't not offer too much centrally. In England's usual shape of a front three, he could be competing for a place on the right, where they look well stocked. The likes of Raheem Sterling, Phil Foden and Jack Grealish are very short in the betting and it is hard to see Greenwood getting ahead of them. Indeed bookmakers also think his United team-mate Jesse Lingard will be selected for the final 26 at a price of 1/8.
Eric Dier is another Southgate favourite whose selection could mean the likes of Alexander-Arnold and Greenwood miss out.
Three centre-halves are short prices to make the squad - two Manchester-based defenders in John Stones of Man City and Harry Maguire of Man United, and Tyrone Mings of Aston Villa. Many believe that Southgate will take a fourth, especially with injury concerns for Maguire, and that likely means a place in the squad for Tottenham's Dier, who has been a constant selection for the England boss.
James Ward-Prowse has been a regular in recent squads. The Southampton skipper was given his first competitive start in March this year, and also scored his first goal in the same month, so the offer of him being 10/11 could provide value.
Everton's Ben Godfrey has been backed into 3/1 and is the best-backed player in recent weeks. His team-mate Michael Keane looks set to miss out, but Godfrey's ability to play across the back-four makes him an interesting option and some believe he could be a credible alternative to Dier. Bookmakers, however, believe that Conor Coady of Wolves is most likely to push Dier for the fourth choice centre-half slot and he is 4/7 to get the nod from Southgate.
James Maddison seemed a certain selection earlier in the season but injuries have seemingly hampered his hopes, although his odds have come in at around 4/1 to make it.
One player who bookmakers don't think is going to make it is 8/1 Danny Ings - but the Saints man is one to keep an eye on, as some think that he has a real chance of making the squad following his return from injury and a return to form.
BetVictor's Head of Ante Post Football Michael Triffitt discussed the England squad with Planet Sport, saying: "Alexander-Arnold was considered a virtual certainty to make the England Euro squad after a strong start to the season but now faces a real battle to make the cut, even with the squad size increasing to 26. After being 1-16 earlier in the campaign he is now just 4-7 as rivals emerge to challenge his place.
"Those holding 8-1 vouchers on Eric Dier will be getting excited as he has contracted to 4-7 and may be one of those to replace Arnold. Ben Godfrey was considered very unlikely to be playing international football in the summer but his odds have shortened dramatically from 33-1 into just 3-1 and he has been the best-backed player to make the squad.
"Manchester United star Mason Greenwood has endured a strange season and has probably underachieved in the main. However, he has shown good form in the latter half of the season and the earlier 6-1 has now fallen to just 4/7 with fans clambering for his inclusion.
"There seems to be plenty of attacking options but for those looking for value, Southampton frontman Danny Ings is available at 8-1 and it wouldn't be the biggest shock if he was selected now.
"Jesse Lingard is another player who now looks likely to be included as his 1-8 odds would indicate. It's hard to believe he was 25-1 when Manchester United allowed him to join West Ham on loan but there are plenty on at fancy prices after backing him to kickstart his international career.
"With the tournament being pushed back a year there are plenty of clients reaping the benefits. Jude Bellingham was considered a 16/1 outsider a year ago but is now 1-3 after a tremendous season at Borussia Dortmund after joining from Birmingham."
Jordan Pickford (1/20)
Dean Henderson (1/16)
Nick Pope (1/8)
Ben Chilwell (1/20)
John Stones (1/20)
Luke Shaw (1/16)
Kyle Walker (1/16)
Harry Maguire (1/5)
Tyrone Mings (2/5)
Reece James (2/5)
Kieran Trippier (1/2)
Eric Dier (4/7)
Bukayo Saka (4/7)
Mason Mount (1/20)
Declan Rice (1/16)
Phil Foden (1/20)
Kalvin Phillips (1/8)
Jack Grealish (1/4)
Jude Bellingham (1/3)
Jesse Lingard (1/8)
Jordan Henderson (4/7)
Jadon Sancho (1/8)
Raheem Sterling (1/25)
Harry Kane (1/25)
Marcus Rashford (1/20)
Dominic Calvert Lewin (1/12)
Just missing out…
James Ward-Prowse (10/11)
James Maddison (4/1)
Mason Greenwood (4/7)
Trent Alexander-Arnold (4/7)
Ben Godfrey (3/1)
Michael Keane (5/1)
Ollie Watkins (8/1)
Conor Coady (4/7)