(a) Written notice of disability shall be furnished to the employer by or on behalf of the employee claiming benefits or in the case of a claimant under section 207 of the Workers' Compensation Law, to the chairman, within 15 days after commencement of the period of disability. Proof of such disability shall be furnished to the employer or carrier or, in the case of a claimant under section 207 of the Workers' Compensation Law, to the chairman, not later than 20 days after commencement of the period of disability and thereafter from time to time as the employer or carrier or chairman may require but not more often than once each week.

(b) Such notice and proof shall include statements furnished by the claimant and the claimant's attending physician, podiatrist, chiropractor or dentist in form prescribed for such purpose by the chairman. The attending physician, podiatrist, chiropractor or dentist shall complete and mail such form to the carrier or self-insured employer, or return it to the claimant, within seven days of receipt of the form.

(c) A carrier, employer or the chairman may investigate a claim and for cause require additional information.

(d) Failure to furnish notice or proof within the time and in the manner above provided shall not invalidate the claim, but no benefits shall be required to be paid for any period more than two weeks prior to the date on which the required proof is furnished. However, if it shall be shown not to have been reasonably possible for the claimant to furnish such notice or proof and that such notice or proof was furnished as soon as possible, benefits may be payable for the full period of disability; provided, however, that no benefits shall be paid unless required proof of disability is furnished within 26 weeks after commencement of the period of disability.