The price of silver continued its recent tear on Monday, climbing 3.6% to $20.29 an ounce, having briefly spiked at above $21 in intraday trading on the Asian market. That gain followed a 6% rise on Friday and means that the world's 'other' precious metal has increased 40% since the start of the year, outstripping gold's 25% increase over the same period.

The rally has taken silver prices beyond $20 for the first times in less than two years and pushed the value of silver producers and silver funds sharply higher. The iShares Silver Trust SLV, the world's largest and oldest silver-based exchange traded fund, traded Friday at $18.74, up 4.9% on the day and 38.9% since the start of the year.

Analysts believe that there may still be room to run. Having broken through the $19.16 barrier last week the next major resistance level is likely to be found at $20.42, Goldman Sachs analysts noted last week.

The increase in silver prices has been fueled by the same factors that have driven gold higher, namely market uncertainty and interest rates.

Britain's vote to leave the European Union delivered uncertainty in spades, encouraging investors to switch into precious metals due to their perception as a store of wealth. Low interest rates further support that shift by reducing the opportunity cost of holding precious metals versus other safe haven assets.

Germany's 10-year bund yield's turned negative for the first time ever on June 14, and has remained there since. Britain's 10-year gilts hit record lows of 0.78% on Friday and are likely to move lower after Bank of England Governor Mark Carney last week warned that the British economy was suffering "post traumatic stress" in the wake of the referendum. The 10-year Treasury yield on Friday briefly matched its all time low of 1.38%, and few analysts now expect the Federal Reserve to increase interest rates this year.

Analysts have responded by upgrading silver producer's price targets, though the speed with which some of the producers' stock has risen have left brokers playing catch up.

Precious metal producer Fresnillo  (FNLPF)  is up 6.10% in London today and is up 50% since the referendum. 

Raymond James last week increased its target price for First Majestic Silver (AG - Get Report) to C$11.25 ($8.74) from C$8, leaving the forecast price well below First Majestic's Friday closing price of C$17.55. First Majestic climbed 9% on Friday.

Silvercorp Metals (SVMLF) , another Canadian silver miner, was upgraded by Raymond James to C$3 from C$2.50, leaving it with a slim upside to its Friday closing price of C$2.94. Goldman Sachs raised its price target for Silver Wheaton (SLW) to C$32, leaving it with a forecast 5% upside from its Friday close of C$30.41.

Silver remains a high volatility investment. The metal traded as high as $40 an ounce in 2011, before falling to nearly $13 in December last year.

There is good reason to expect that volatility will continue - particularly as much of the current support is from financial backers rather than fundamental demand. Imports of silver into India, the world's No.2 user of the metal after the U.S., have ground to a halt in recent weeks as local traders, who bought silver earlier in the year, sell off large stockpiles at prices below the import price, according to Indian newspaper Business Standard.