European stocks rallied the most in a month after European Central Bank President Mario Draghi said policy makers agreed to an unlimited bond-purchase program as they try to regain control of interest rates in the euro area.

The ECB program “will enable us to address severe distortions in government bond markets which originate from, in particular, unfounded fears on the part of investors of the reversibility of the euro,” Draghi said at a press conference in Frankfurt. “Under appropriate conditions, we will have a fully effective backstop to avoid destructive scenarios with potentially severe challenges for price stability.”

National benchmark indexes climbed in all 18 western- European markets, and all 19 industry groups in the Stoxx 600 advanced. Germany’s DAX rallied 2.9 percent and the U.K.’s FTSE 100 gained 2.1 percent. France’s CAC 40 jumped 3.1 percent.

A gauge of banks had the biggest gain among 19 industry groups in the Stoxx 600, rising 4.4 percent. UniCredit, Italy’s largest lender, advanced 8.1 percent to 3.51 euros and France’s Societe Generale rallied 7.8 percent to 22.93 euros. Banco Espirito Santo SA surged 8.3 percent to 61.3 euro cents in Lisbon.

Nokia Oyj, the mobile-phone maker that lost more than 95 percent of market value since 2007, dropped 2.9 percent to 1.93 euros after the stock was cut to sell from hold at Deutsche Bank AG. The shares sank 13 percent yesterday after the company unveiled two smartphones using Microsoft Corp.’s new Windows Phone software.