Image copyright Reuters Image caption Bitcoin's value has been very volatile during its nine-year history

Digital currency Bitcoin has fallen 30% this week, leaving it on track for its worst week since April 2013.

On Friday the price fell below $7,910 on the Bitstamp exchange, a 12% fall on the day before, but recovered slightly.

But although it is far short of the $19,000 it reached in November 2017, it is still way above the $1,000 level at which it started trading last year.

The fall comes amid a number of recent incidents that appear to have shaken faith in cryptocurrencies.

On Friday, Japan's financial regulator carried out a surprise check on major Japanese exchange, Coincheck, which last week was subject to a security hack.

The regulator said it had asked the exchange to fix flaws in its computer networks well before last week's theft by hackers of $530m of digital money.

Also this week, Facebook said it would ban adverts for digital currencies.

Other countries have already expressed concerns about such entities. China and South Korea have banned any new virtual currency launches and have been shutting down exchanges on which they are traded.

The UK's Financial Conduct Authority warned investors in September they could lose all their money if they buy digital currencies issued by firms, known as "initial coin offerings".

Bitcoins are created through a complex process known as "mining", and then monitored by a network of computers across the world.

However, like all currencies its value is determined by how much people are willing to buy and sell it for.

Last year, two of the world's largest commodity exchanges, the CBOE futures exchange and the Chicago Mercantile Exchange, both allowed trading in Bitcoin futures.

Bitcoin is the most widely traded cryptocurrency, but there are scores of others, the majority of which also fell on Friday, according to the Coinmarketcap.com price tracker.

The second and third largest virtual currencies, Ethereum and Ripple, plunged more than 20%, before a slight pull-back in the price.