Wal-Mart Stores Inc is planning to test a new unlimited online shipping service this summer for USD 50 per year, a move that may hurt Amazon's annual USD 99 Prime shipping service.

Wal-Mart's service, which will be by invitation only for now, will offer selected products on the company's website to customers within three days or less, company spokesman Ravi Jariwala told Reuters. Wal-Mart offers more than 7 million products on its website.

"Depending on customer feedback we will see how the programme evolves," Jariwala said.

The move underscores Wal-Mart's efforts to scale its online business rapidly and gain a share of a market dominated by Amazon .

Amazon's launched its USD 99-a-year shipping service called Prime, a decade ago with the guarantee of standard, reliable two-day shipping on online orders.

Since then Prime has become the cornerstone of Amazon's growth and the e-commerce company said US Prime memberships grew 50 percent last year.