If you thought your inbox was looking a little less full of unsolicited invitations to improve your sex life or boost the size of your manhood lately, you are not alone.

Recent months have witnessed a precipitous decline in the amount of spam email being sent out worldwide. One leading security firm has estimated that while 200 billion spam messages were being sent out each day during the summer, this figure has plummeted to "just" 50 billion since Christmas.

"We do see spikes and falls in the volume of spam on a daily basis but what we haven't seen before is a drop of this magnitude, which is unexpected and unexplained," said Paul Wood, a senior analyst at the security firm Symantec who highlighted the change.

Experts have observed that one of the internet's most prolific spammers, known as Rustock, has effectively ceased its activities. At its peak Rustock – which took its name from a scam offering Russian stocks – was firing out 44 billion spam emails every 24 hours, accounting at some points for about half of all spam sent. This has now declined to just 100 million messages a day.

The reduction, however, is not the result of a law-enforcement operation. In 2008, attempts were made to disrupt the command and control centres through which spammers illegally control vast networks of infected PCs which send out the unwanted emails on their behalf – so-called botnets.

The sudden silence from Rustock coincides with two other smaller botnets – Lethic and Xarvester – also going strangely quiet.

Some analysts fear this may mean that the spammers are gearing up for a new assault on the world's inboxes. "We are yet to see what that might be," Mr Wood said.

Spam, an industry said to rival the illicit drugs trade with an estimated value of $100bn (£65bn) a year, last year accounted for about 90 per cent of all email sent – although the vast majority is filtered out by anti-virus software.