Follow the link to FA site for ICT guidelines.

In accordance with FIFA Regulations, international clearance is required for any player over the age of 10 who is seeking to register for a club in England and has previously been registered for a club abroad (including the other Home Associations), or who is a foreign national seeking to register for the first time in this country.

FA Standard Code of Rules - 18 (b) (i) also refers

http://www.thefa.com/football-rules-governance/policies/player-registration/international-clearance