5. The Commission is empowered to adopt delegated acts in accordance with Article 28 in order to supplement this Regulation by establishing further methods and criteria of accreditation of verifiers, on at least the following elements: request for accreditation for verification activities within the scope of this Regulation; assessment of verifiers by the national accreditation bodies; surveillance activities performed by the national accreditation bodies to confirm the continuation of the accreditation; administrative measures to be adopted if the verifier does not fulfil the requirements of this Regulation; and requirements for national accreditation bodies in order to be competent to provide accreditation to verifiers for verification activities within the scope of this Regulation, including a reference to harmonised standards. The methods and criteria specified in such delegated acts shall be based on the principles for verification provided for in Articles 11, 12 and 13 and on relevant internationally accepted standards.