Fears that there was not enough gold in New York vaults to deliver against futures contracts pushed prices on CME Group's Comex exchange as much as 4% above spot prices in late March.

The huge price gap created an incentive to ship gold to New York, often via Switzerland - a key trading, vaulting and refining centre for precious metals.

Switzerland exported 361 kgs to the United States in February. The March number is the biggest since Swiss data began in 2012 and equivalent to all its gold exports to the United States in the previous 40 months combined.

Switzerland also imported 28.2 tonnes of gold from Britain in March, up from 163 kg in February and the most for any month since May 2019.

London is another vaulting centre and the biggest trading hub for physical gold. However, it uses 400-ounce gold bars, while the Comex exchange has a unit size of 100 ounces.

This difference means bars from London must be recast, usually in Switzerland, before shipping to the United States for Comex.

The difficulty of doing this when coronavirus lockdown measures grounded planes and closed several Swiss refineries in March was a key factor pushing Comex futures above spot prices.

The surge in deliveries to the United States offset a continuing collapse in Switzerland's gold exports to India and China, usually its largest markets with hundreds of tonnes of imports a year.

Swiss exports to mainland China fell to zero in March for the first time since September 2012. It shipped 152 kgs to Hong Kong and 6.6 tonnes to India, which entered coronavirus lockdown later than China.

In total, Switzerland exported 96.1 tonnes of gold in March, up from 42.8 tonnes in February.

(This story has been refiled to correct month in final paragraph).

By Peter Hobson