England fans flying the cross of St George on their cars could antagonise Welsh fans, according to the deputy chief constable of North Wales Police.
Many England fans are flying the flag on their cars for the World Cup
Clive Wolfendale said "incessant" flag-waving in Wales during the World Cup could lead to racism and violence.
His comments were made during a launch of the force's anti-violence campaign to coincide with the World Cup.
Clwyd West MP David Jones said flags were "innocuous" and that Mr Wolfendale was "completely misguided".
Mr Wolfendale, who is English, said there is a place for flag-waving during the World Cup.
But he believes that in the weeks before the tournament, the sight of the English flag being displayed on cars "frequently and overtly" is "unnecessary".
'In your face'
Mr Wolfendale said: "I'm an Englishman and I'll be supporting England during the World Cup campaign and if I was at the game or in and around the stadium there's a fair chance I might be waving the flag.
"But I think there comes a point when weeks before the tournament the incessant use of these symbols is a bit in your face and I just wish people would sometimes reflect on the impact that's having on others."
Clive Wolfendale said overt flag-waving was "unnecessary"
Mr Wolfendale said that in a football context Wales was "definitely another country" adding that he wished England fans "would reflect on that before they start festooning their vehicles in this way".
He said that displaying the flag was not illegal but that in Wales it was a "matter of courtesy and of decent behaviour" for England fans not to do so.
He added: "There's no doubt about it that this can be the precursor to behaviour which is much worse than flag waving - it's violence, it's racism, it's hooliganism of the very worse kind."
North Wales Police have launched a pub campaign entitled Show Violence the Red Card.
Posters carrying the slogan have also been printed in Polish and Portuguese - two countries also competing in the World Cup, and which have large communities in Wrexham.
Mr Wolfendale's comments have been criticised by some MPs in north Wales.
Alyn and Deeside Mark Tami said the remarks were not helpful and risk creating a problem "that isn't actually there".
He said he does not believe that tension between English and Welsh fans exists in north Wales.
Clwyd West MP David Jones said Mr Wolfendale's comments were "laughable".
He added: "I think it's pretty innocuous (for fans) to display their flag from the little flagpoles on their cars if they want to do that.
"I can't believe that the people of Wales are so intolerant of a few English football fans that they would be incited to violence - I think he's completely barking up the wrong tree.
"Frankly, flying a little flag on your way to Colwyn Bay for the day is, I don't think, going to spark any violence - I think he's completely misguided."