Except as provided in subsection (c) of this section, any one or more contributing sponsors who withdraw, during a plan year for which they constitute a substantial employer, from a single-employer plan which has two or more contributing sponsors at least two of whom are not under common control, shall, upon notification of such contributing sponsors by the corporation as provided by subsection (a) of this section, be liable, together with the members of their controlled groups, to the corporation in accordance with the provisions of section
1362 of this title and this section. The amount of liability shall be computed on the basis of an amount determined by the corporation to be the amount described in section
1362 of this title for the entire plan, as if the plan had been terminated by the corporation on the date of the withdrawal referred to in subsection (a)(1) of this section multiplied by a fraction—
In addition to and in lieu of the manner prescribed in the preceding sentence, the corporation may also determine such liability on any other equitable basis prescribed by the corporation in regulations. Any amount collected by the corporation under this subsection shall be held in escrow subject to disposition in accordance with the provisions of paragraphs (2) and (3) of subsection (c) of this section.