Online banking customers are being advised to stay vigilant after being targeted by a surge in hoax e-mails.

Reported cases of fraudsters trying to steal bank details using bogus e-mails more than doubled in the first half of 2008 compared with 2007.

Apacs, the UK payments authority, said there were 20,682 "phishing" frauds involving banks and building societies reported in the first six months of the year, against 7,224 a year earlier.

Sandra Quinn at Apacs says: "We strongly urge banking customers to make sure they remain wary of online scams such as unsolicited emails claiming to be from their bank, and to only use a fully protected PC with regularly updated anti-virus software and a firewall installed and switched on."

A phishing attack involves criminals sending bogus e-mails purporting to be official requests. These ask customers to confirm their online banking details by e-mail or by directing them to a website that looks just like the bank's own site. The e-mails look genuine and will often even include advice on how to avoid fraud.

Spyware programs, capable of monitoring computer activity, potentially capturing security details such as user names and passwords, are another favourite scam. Usually the criminals send out e-mails at random to get people to click on a link and visit a website that installs the virus.

Despite the warning, Apacs said that losses to online banking fraud have been falling. Fraud cost banks and building societies £22.6m in 2007, according to Apacs, a 33% decrease on the year before.

The number of people banking online has soared by 500 per cent during the past seven years to just over 21 million people in 2007.

FIVE TIPS TO PROTECT AGAINST FRAUD

* Install an anti-virus system and firewall on your computer and keep them up to date. A message will appear on your screen when updates need downloading.

* Experts do not recommend you click on any link in an e-mail, even if it seems genuine. Type in the web address, just in case. If in doubt, contact the bank using an advertised phone number. Don't try and hit back at fraudsters by replying to phishing emails and providing bogus information or letting the sender know that you are aware it is a scam. Doing so puts your PC at risk.

* Do not divulge Pins or passwords to anyone. Because fraudsters only have very limited information, phishing emails are usually adressed to "Dear valued customer" rather than to you personally.

* Avoid obvious passwords such as your child's name or your date of birth.

* Only make transactions on secure websites that begin 'https' or display a padlock icon in the corner of your web browser.