Security researcher Scott Helme said more than 4,000 websites, including many government ones, were affected.

The cryptocurrency involved was Monero - a rival to Bitcoin that is designed to make transactions in it "untraceable" back to the senders and recipients involved.

The plug-in had been tampered with to add a program, Coinhive, which "mines" for Monero by running processor-intensive calculations on visitors' computers.

Once the plug-in was infected, it affected thousands of other websites in addition to the ICO's, which used it.