A new application form CA8421(A) is available for International Transport workers when they are working in two or more EEA countries.
As a general rule, a person is insurable for national insurance purposes in one EEA State only. Generally, employment is insurable in the State in which it takes place. However, if an employee who is posted from the UK to work in another EEA State, then he/she may be entitled to apply for an E101 Certificate under EU Regulations. The form E101 is proof that the employee remains within the UK National Insurance system and that the employee will not be required to pay National Insurance or similar contributions in other EEA countries.
The new form is available here