MSC initially declined to comment when contacted by Reuters. After the news was published, it said in a statement: "Delivery of shipments to our customers are on schedule and our operations are business as usual. We will investigate this matter and will provide an update where necessary."

Tin is a vital ingredient for electronic components for mobile devices, electric vehicles, robotics, renewable energy and energy storage industries. It is also used to make chemicals and coat other metals to prevent corrosion.

MSC's refined tin production last year was 27,085 tonnes, according to its annual report.

It is the world's third-largest refined tin producer after China's Yunnan Tin Co Ltd and Indonesia's PT Timah.

One source said MSC had told customers that deliveries could take two-three weeks instead of the usual three-four days and that shipments had been delayed since February.

"MSC is short of concentrate and has pushed some delivery schedules out by four weeks or so," another source said. "The problem has been brewing for weeks."

Sources say tin concentrate supplies from Nigeria and the Democratic Republic of Congo have declined this year due to lower prices, while supplies from Myanmar have been on a downtrend for some time due to depletion of easy-access reserves.

Refined tin exports from Indonesia rose in April but over the past couple of years they have been erratic.

Nervousness about shortages pushed benchmark tin prices on the London Metal Exchange to $19,500 a tonne on Friday, their highest in more than a week and up more than $900 a tonne, or about 5%, since May 31.

One base metals trader said tin supplies to Korea, Japan and some parts of Europe were affected. "(It) may well be the reason for the big backwardation in the tin market for the past two weeks," the trader said.

The premium or backwardation for the cash over the three-month tin contract is around $130 a tonne, up from $60 a tonne in the middle of April. It touched $340 a tonne last month, the highest since September 2015.

(Reporting by Pratima Desai and A. Ananthalakshmi; additional reporting by Mai Nguyen; editing by Veronica Brown, Subhranshu Sahu and Dale Hudson)

By Pratima Desai and A. Ananthalakshmi