PayPal said in January that it was launching beta trials of the virtual card payments system in partnership with MasterCard.

According to a Reuters report, PayPal's new Secure Card plug-in has been tested by three million customers and will be available to US consumers from Tuesday. An international roll out is expected to follow.

The browser plug-in detects when a user lands on an e-commerce checkout page and automatically fills in their stored financial details.

The system also generates a Mastercard number and one-time card verification code that are linked to PayPal accounts and used when paying for online purchases at Web sites that don't accept PayPal. Using technology from Orbiscom, the virtual card generates a unique number for each new purchase.

Chris George, director of financial products, PayPal, told Reuters reporters that the transaction looks like any other MasterCard payment to the merchant and a normal PayPal purchase to the customer.

As well as increasing convenience for customers, the plug-in will multiply the number of Web sites that can offer the PayPal service by extending the system to merchants that don't currently have the PayPal technology embedded in their own e-commerce services.

Secure Card is a response to the Google Checkout Web payments processing service which was launched last year. Checkout also stores customers financial details to make shopping online quicker and more convenient for customers.

However despite the competition PayPal has continued to grow and now claims 164 million user accounts worldwide.

A study released by Javelin Research & Strategy earlier this year predicted further growth for PayPal as consumers increase take up of "alternative payment methods" - such as e-mail payment accounts.