A number of UK High Street banks have stopped trading in Qatari riyals.
Individual customers at Barclays, RBS, Lloyds Banking Group and Tesco Bank cannot currently buy or sell riyals.
Qatar is isolated by its neighbours, who accuse it of backing terrorism.
Its central bank says it will guarantee all transactions for customers inside and outside the country. It also told the Reuters news agency that all banks and foreign exchange companies are committed to trading riyals as usual.
In early June, Qatar’s Gulf neighbours Saudi Arabia, UAE, Bahrain and Egypt ceased air, sea and land links with the country, which has been accused of funding terrorism. Qatar denies the claim.
The cutting of diplomatic ties by Qatar’s neighbours has prompted wide fluctuations in the value of the country’s currency.
A spokeswoman for Barclays said: “In common with other banks, Barclays’ high street foreign exchange service is supplied by a third party, which has stopped providing the Qatari riyal.
“Unfortunately, we are therefore currently unable to buy the Riyal from or sell it to our retail customers.”
A spokeswoman for Lloyds Banking Group also said that a “third-party supplier” which carries out its foreign exchange service had ceased trading Qatar riyals from 21 June.
She said: “This currency is no longer available for sale or buy-back across our high street banks including Lloyds Bank, Bank of Scotland and Halifax.”
HSBC was unavailable for comment on its current position. Reuters reported that the Post Office had stopped buying and selling the currency earlier this month.
Currency supplier Travelex said trading in riyals had been suspended in some markets due to “business challenges”.
But it added: “Travelex is pleased to announce it has resumed purchasing Qatar riyal globally.”