The model worn by Essex's Cook in the Division Two win over Gloucestershire has an adjustable grille and a large gap between peak and grille.
This contravenes new regulations brought in following facial injuries to Stuart Broad and Craig Kieswetter.
Cook could face censure if he continues to wear the old helmet.
Umpires have been told not to allow players on to the field with non-compliant helmets, and they can also refer the case to the cricket discipline commission.
Cook has been made available for Essex's first four County Championship matches before he is due to captain England in the first Test of the summer, against Sri Lanka at Headingley on May 19.
Cook's Essex team-mate Ravi Bopara told Sky Sports News the new design was hard for batsmen to get used to.
"It is tough to start using those helmets - you do lose the ball for that less than a split-second, and that makes a whole load of difference to a lot of players," he said.
"I understand where Cookie's coming from. But when it comes to the guidelines and safety, we all have to be aware of that."
England captain Alastair Cook has been spoken to by the England and Wales Cricket Board (ECB) following his decision to defy new helmet rules.