The world's biggest online payment service, PayPal, has said it will block "unsafe browsers" from using its service as part of anti-phishing efforts, likening not stopping customers from using old software to letting a car owner drive without a seatbelt.

PayPal said it was "an alarming fact that there is a significant set of users who use very old and vulnerable browsers such as Internet Explorer 4" and that some customers are even still using Internet Explorer 3, released more than 10 years ago.

Under the new plans, unveiled in a white paper, customers will first get a pop up warning that a certain browser is unsafe but could then be blocked if they continue.

PayPal's chief information security officer recently attacked Apple's Safari browser for not having two important anti-phishing security features.

"Apple, unfortunately, is lagging behind what they need to do, to protect their customers", the officer said. "Our recommendation at this point, to our customers, is use Internet Explorer 7 or 8 when it comes out, or Firefox 2 or Firefox 3, or indeed Opera."