(Reuters) -Silver prices surged to their highest in almost 14 years on Wednesday, aided by worries about U.S. tariff policy, signs of tightness in the spot market and growing investor interest in alternatives to gold.

Spot silver was up 0.3% at $39.40 per troy ounce as of 1354 GMT, its highest level since September 2011. 

Silver, both a precious and industrial metal, is up 36% this year, outperforming gold's 31% growth and coming within a whisker of the key $40-per-ounce mark. The metal hit a record high of $49 in 2011.

U.S. President Donald Trump's plan to impose 50% import tariffs on copper from August 1 and the U.S. import tariffs for Mexico widened the premium of the U.S. futures for silver and other metals against the London benchmarks in July, leading to a growth in lease rates in the spot market. 

Gold, silver, platinum and palladium were excluded from Trump's April reciprocal tariffs, but "the broader market isn't trading it that way and is taking a page out of Comex copper's handbook", Nicky Shiels, head of metals strategy at MKS PAMP.

Spot silver prices may hit $42 per ounce this year, according to Shiels.

Analysts also noted that industrial demand for silver, heading for the fifth year of structural market deficit, remains solid, while investment demand is gaining momentum as a more affordable alternative to gold.            

Silver's recent rally has improved its ratio with gold to the strongest level in seven months. It currently takes 87 ounces of silver to buy an ounce of gold, compared with 105 ounces in April.    

"It is the copper tariff that sent some spinning off at odd tangents that captured the other metals," said a precious metal trader based in London, adding that the lease rates in the spot market should fall once the borrowing activity caused by the U.S. tariff fears subside.

The current momentum could be hot enough to take silver over $40/oz in the short term, said Nitesh Shah, commodity strategist at WisdomTree.

"But with positioning stretched, we would not be surprised if it fell back to $35/oz, before it starts its march higher to $45/oz next year," Shah added.