Apple Watch wearers may be in for a surprise with rumours that it has a secret port for charging, Digital Spy reports.

A company behind the upcoming Reserve Strap for the Apple Watch discovered an opening which is covered by a metal door on the device.

The Verge quotes Reserve Strap as saying that its diagnostics tests determined that this secret port can actually charge at a faster rate than the Apple Watch's inductive charger.

The company said: "By utilising this port instead of wirelessly charging, we've been able to achieve a higher charge capacity and quicker, more efficient charging times while also improving durability and eliminating any interference with Apple Watch functionality including taptic feedback and heartrate sensors."

It is unclear if Apple is intending for customers to use this extra port. If not, it could disable its use with a Watch OS firmware update in the future.

The Apple Watch began shipping to customers at the end of April, eight months after it was first unveiled by Tim Cook at a September 2014 media presentation.