Amazon has built a wall around its secret drone test site in Cambridgeshire to stop people looking in

Amazon has constructed a giant wall on a farm in Cambridgeshire to conceal its largest outdoor drone testing site in the world, MailOnline reports.

Photos published by MailOnline suggest the walls are around three to four metres tall, which is considerably higher than the average man.

The construction of the walls comes less than two weeks after Business Insider revealed the exact location of the drone test site.

The site - located on a private farm about eight miles south-east of its main R&D facility in Cambridge - is also patrolled by security guards. The guard we spoke to denied any knowledge of a drone testing site when we confronted him at the site. MailOnline reports that there are vans patrolling the site too.

Amazon founder Jeff Bezos revealed plans for Amazon Prime Air in an interview on "60 Minutes" in December 2013. The conceptual drone-based delivery system is currently in development and a number of drone designs have been released.

Amazon wants to use the drones to deliver packages to people's homes and offices in under 30 minutes. It claims the drones will be greener, cheaper, and safer than the vans that are currently used to deliver Amazon packages.

While Amazon has released videos and photos of what its delivery drones might look like, the Seattle-headquartered company has been very secretive about its tests.

Daniel Buchmueller, the cofounder of Prime Air, Amazon's drone business, told journalists at a press event last month that the company's largest outdoor drone test site was somewhere in the UK, without specifying where. The company's PR team has repeatedly refused to give away any more information than that.

The Civil Aviation Authority (CAA), which regulates drone flying in the UK, has also refused to reveal where it has given Amazon permission to test its drones.

The test site includes a single container at one end of a field and a slightly larger blue complex at the other end, complete with a tower and aerials. The bases are roughly 400 metres apart and are likely used to launch and house the drones.

MailOnline reports that the test site represents a multi-million-pound investment by Amazon although it's unclear what evidence there is for this.

Amazon did not immediately respond to Business Insider's request for comment.