Amazon says its free shipping for Prime members will be even faster, promising one-day shipping on most items instead of two-day delivery.

The company didn't say when the change would happen, but says it has been growing its warehouses and shipping network to offer faster deliveries.

Amazon has long offered delivery in a day or quicker for some items, but the change will mean free one-day shipping will be the default, rather than two-day.

The move is likely to put even more pressure on big retailers, which have been racing to ship faster to catch up with Amazon.

Amazon says it will offer one-day shipping to its Prime members around the world.