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Gold posted the longest run of weekly gains in almost two years and silver surged to the highest since 2014 as Britain's vote to quit the European Union fueled speculation that central banks will boost economic stimulus.

Havens such as gold and silver are in demand on prospects for weaker economies.

Treasury yields fell to record lows along with sovereign rates from Spain to Japan as policy makers signaled their readiness to shore up economies. Gov. Mark Carney said Thursday that the Bank of England could cut interest rates within months.

Gold and silver had the biggest first half gains in about four decades amid mounting speculation that interest rates in the United States will remain low, which is a boon to precious metals because they don't offer interest.

Expectations for U.S. rate increases have been wound back since the Brexit vote, while investors continue to pile into exchange-traded funds backed by gold.

"Every hint at lower interest rates and at uncertainty is propelling gold and silver higher, whether that is rational or not," said Thorsten Proettel, a commodity analyst at Landesbank Baden-Wuerttemberg in Stuttgart. "The governor of the Bank of England was the latest to play into this narrative."

Gold futures for August delivery advanced 1.4 percent to settle at $1,339 an ounce at 1:55 p.m. on the Comex in New York, capping a fifth straight weekly rise. Prices rallied 25 percent in the first half, the biggest such gain since 1979. For spot gold, the rally in the first half was the largest since 1974.

Silver futures for September delivery climbed 5.2 percent to $19.588 an ounce. In electronic trading after the market closed, the metal touched $19.745, the highest since August 2014.

"What will drive the price of gold going forward is that by how much these central banks will ease their monetary policy, and that will serve as a catalyst for further movement," said Naeem Aslam, chief market analyst at Think Forex U.K. Ltd.

Traders are pricing in a less-than 50 percent chance of higher U.S. rates by the end of 2017, according to Fed funds futures.

In other metals:

• Assets in gold-backed ETFs are at the highest since 2013.

• Platinum and palladium futures also advanced on the New York Mercantile Exchange.