Shoppers have been temporarily banned from using £2 coins at self-service tills in Morrisons supermarkets after some people were found to have been paying with foreign coins of a similar size.

Morrisons said it would not accept £2 coins at the self-service checkouts of certain stores until an upgrade had been carried out so that foreign substitute coins, which are often worth far less, could be recognised.

It said a "handful" of stores were affected and it did not expect the upgrade to take long. Customers are still free to pay with £2 coins at manned checkouts.

At first, only stores in the West Midlands were said to be affected, but it is now thought that the problems may be more scattered. Some Yorkshire-based Morrisons customers have reported seeing signs placed in store about the issue.

Morrisons declined to say exactly how many stores were affected or where they were, citing security and commercial reasons.

A spokesman said: "A handful of our stores have experienced foreign currency being passed off as £2 coins at self checkouts. While we fix the issue, some of those stores have temporarily stopped them accepting any £2 coins.

"Customers are still free to use them at all manned checkouts. We apologise for any inconvenience it is causing but don't expect the fix to take long."

Coins with similar characteristics to the £2 coin include the Iranian 250 rial, which is worth around one pence, and the Thai 10 baht, worth about 18p. The €2 coin, worth around £1.60, also has a similar appearance.

Jonathan Hilder, CEO of the Automatic Vending Association (AVA), said that in general machines recognised coins by their shape. He said that to his knowledge there had been no widespread problems with machines failing to properly recognise the £2 coin. "Being a bi-metal coin, [it] is particularly difficult to forge," he said.

This year the Treasury unveiled plans for a new 12-sided £1 coin with a bi-metallic makeup similar to the £2, to be introduced in 2017.