LONDON (Reuters) - London spot gold prices fell far below U.S. gold futures on Tuesday in a sign the market is worried air travel restrictions and precious metal refinery closures will hamper shipments of bullion to the United States to meet contractual requirements.

The promise of unlimited stimulus by the U.S. Federal Reserve on Monday sent gold prices soaring, but London’s spot market has started lagging behind prices on the Comex futures exchange in New York.

At 1516 GMT, spot gold was up 3% at around $1,600 an ounce, while March futures on the Comex exchange were up nearly 5% at $1,642 an ounce - a price difference of $42.

These prices normally trade within a few dollars of one another. At one stage on Tuesday the difference was more than $70. On Monday, it was around $15.

The higher New York price reflected the perceived cost of taking metal from London to deliver against Comex futures contracts in the United States, traders and bankers said.

London is home to thousands of tonnes of gold which underpin the world’s largest hub for physical gold trading.

But if physical gold from London is needed to deliver against Comex futures contracts it has to be melted down from the 400 ounce bars used in London and recast as 100 ounce bars accepted by Comex.

This has suddenly become more difficult as governments restrict movement of people and goods and after three major metals refineries in Switzerland suspended operations on Monday.

“People are worried about whether they can get the gold to deliver into the futures contracts,” said a bullion banker.

The London Bullion Market Association (LBMA), which oversees the London trade, said price volatility in New York had impacted liquidity in London and it had “offered its support to CME Group to facilitate physical delivery in New York.”

CME Group operates the Comex exchange.

Liquidity on spot gold and the contract used to bridge the London and New York prices - known as an exchange-for-physical (EFP) contract - was in short supply, traders said.

The spread between offered buy and sell prices for spot gold - normally below 50 cents - also rose to as high as $50 on some trading platforms run by banks and brokers on Tuesday, traders said.

One banker said this was in part because trading systems factored the price of the EFP contract into the spread between bids and offers.

The difference between spot prices and U.S. futures was likely to remain in place until either refineries reopened and transport resumed or Comex changed its rules to allow 400 ounce bars to be used to settle its contracts, said a banker at a major gold-trading bank.

“There’s plenty of gold,” the banker said. “But it’s immobilized.”