Jan 15 (Reuters) - Amarin Corp Plc said U.S. health regulators delayed their decision to reconsider a rescinded agreement that could support a marketing application for an expanded use of the company's blood fat-lowering drug.

The U.S. Food and Drug Administration had in October revoked a Special Protocol Assessment (SPA) agreement covering a large late-stage trial of the drug, Vascepa. Following an appeal from Amarin, the regulator said it would determine by Jan. 15 whether it would reconsider that decision.

Shares of the Irish drugmaker, which closed up 15 percent on the Nasdaq on Wednesday in anticipation of a decision, fell 6 percent in trading after the bell.

Amarin said on Wednesday that the FDA in its communication provided no definitive date for its planned response, but that the company does not expect the delay to be significant based on its dialogue with the agency.

The SPA was revoked after an advisory panel to the FDA had recommended against its approval for use in a broader patient population until results from the larger trial had been analyzed.

The regulator determined then that a substantial scientific issue essential to determining the effectiveness of Vascepa in the expanded population was identified only after the trial began.

Vascepa was approved in 2012 to reduce high levels of triglycerides - a type of blood fat that can increase the risk of heart disease - in patients not taking cholesterol-lowering statins such as Pfizer Inc's Lipitor.

In a bid to broaden the drug's market and improve sales, Amarin applied last February for approval to sell Vascepa to patients with blood fat abnormalities who are at high risk of coronary heart disease and are also taking statins.