Supporters who bought their tickets from the allocation given to the respective FAs will be in segregated areas at Lens' Stade Bollaert-Delelis.
But police expect British-based fans to have bought the bulk of the remaining tickets, which are not segregated.
They appealed for fans to behave while in Lens for the match on 16 June.
The stadium has a capacity of 35,000, the second smallest in the tournament.
The Football Association of Wales (FAW) was allocated just over 5,000 tickets and England's FA just over 6,000.
Supt Steve Furnham, of South Wales Police, who is in charge of Wales' police liaison for the tournament, said he expected that "the vast majority of people inside that stadium will be British nationals supporting either England or Wales".
He said he had full confidence in both the French police to cope, and in the fans themselves.
"The French police have got tried and tested procedures and they are well-versed in policing operations and football operations," he told BBC Radio Wales' Good Evening Wales.
"Lens has held games there before and this is not new to our colleagues in Lens or Lens itself, but obviously with thousands of British nationals descending on that town we can expect to see people mingling."
Supporters travelling to France for the tournament can also expect far higher levels of security following the terrorist attacks in Paris and Brussels and Supt Furnham asked for that to be borne in mind.
"What Welsh fans must have in their mind is that the French police are there for the football, but also there for the counter terrorism operation and that will be at the forefront of their mind," he said.
"What we would please, please ask Welsh fans is to behave whilst out there.
"What we do not want to be doing is giving our French colleagues any additional concern or worries on top of what they have got to deal with."
He also revealed that bags will be banned from Euro 2016 stadia.
Similar measures were in place for France's Six Nations matches earlier this year at the Stade de France in Paris.
The Mayor of Lens previously urged fans without tickets to stay away from the town on the day of the match itself.
Thousands of rival Wales and England fans are likely to be sat together in the French stadium hosting their Euro 2016 group match next month.