29 C.F.R. § 4245.3

(a) Requirement of notice. A plan sponsor of a multiemployer plan in reorganization that determines under section 4245 (b)(1), (d)(1) or (d)(2) of ERISA that the plan's available resources are or may be insufficient to pay benefits when due for a plan year shall so notify the PBGC and the interested parties, as defined in paragraph (d) of this section. A single notice may cover more than one plan year. The notices shall be delivered in the manner and within the time prescribed in this section and shall contain the information described in § 4245.4.

(b) When delivered. A plan sponsor shall mail or otherwise deliver the notices of insolvency no later than 30 days after it determines that the plan is or may become insolvent, as described in paragraph (a) of this section. However, the notice to participants and beneficiaries in pay status may be delivered concurrently with the first benefit payment made more than 30 days after the determination of insolvency.

(c) Methods of delivery. The notice of insolvency shall be delivered by mail or by hand to the PBGC and the interested parties described in paragraph (d) of this section, other than participants and beneficiaries who are not in pay status when the notice is required to be delivered. The notice to participants and beneficiaries who are not in pay status shall be provided in any manner reasonably calculated to reach those participants and beneficiaries. Reasonable methods of notification include, but are not limited to, posting the notice at participants’ worksites or publishing the notice in a union newsletter or in a newspaper of general circulation in the area or areas where participants reside. Notice to a participant shall be deemed notice to that participant's beneficiary or beneficiaries.

(d) Interested parties. For purposes of this part, the term “interested parties” means—