New York state filed a $250 million fraud suit Wednesday against H&R Block Inc., charging that the nation’s largest tax preparation service steered more than 500,000 customers into a money-losing retirement account plan.

H&R Block defended the plan and said it would fight to see that it remained available to its clients “who rely on it as a helpful savings option.”

But its shares sank more than 6 percent on news of the lawsuit filed by Atty. Gen. Eliot Spitzer as the tax preparation season is in full swing.