In the event that during the two fiscal years beginning June first, one thousand nine hundred thirty-nine, any act imposing any tax for State purposes shall be held to be unconstitutional by the court having final jurisdiction of such question, appropriations, other than those for the payment of the principal and interest of obligations of the Commonwealth, shall be abated proportionately to the extent necessary to offset any loss of revenue resulting from the invalidation of any such act, as indicated by the official estimate of revenue by the Department of Revenue for the aforesaid two fiscal years: Provided, however, That no preferred appropriation shall abate unless the aggregate proportionate abatements of non-preferred appropriations shall be insufficient to offset such loss of revenue. The Governor shall determine and certify to the fiscal officers the estimated loss of revenue involved and the proportionate amounts by which appropriations are abated under the provisions of this act.