A design feature that could potentially allow some encrypted messages to reach unintended recipients is present within the WhatsApp messaging service.

Facebook-owned WhatsApp, which has about one billion users, has not made it widely known that there is an aspect of WhatsApp that results in some messages being re-encrypted and resent automatically, without first giving the sender an opportunity to verify the recipient.

Campaigners have expressed concern about how this aspect of WhatsApp could potentially be exploited to conduct surveillance.

WhatsApp has made privacy and security a primary selling point, and has become a go-to communications tool of activists, dissidents and diplomats.

Its end-to-end encryption relies on the generation of unique security keys using the acclaimed Signal protocol, developed by Open Whisper Systems. Keys are exchanged between users to guarantee communications are secure from interception by middlemen.

The way WhatsApp implemented the protocol, new keys are generated when – for example – a user gets a new phone or reinstalls the app. Messages for the user which may have been waiting to be delivered while the user was offline are then re-encrypted and resent by the sender automatically, without the sender having had an opportunity to verify that the recipient is the person intended to receive the message. A sender is notified after the event if the sender has opted to turn on a notification in settings, but not otherwise.

This aspect of WhatsApp, which increases convenience and reliability of message delivery at the cost of some security, is not inherent to the Signal protocol. Open Whisper Systems’ messaging app – also called Signal – works differently.

If a recipient’s security key changes while offline, an in-transit message will fail to be delivered and the sender will be notified of the change in security keys without the message having been resent automatically. This approach is known as “blocking”; the WhatsApp approach is called “non-blocking”.

WhatsApp’s online explanatory material states in part: “At times, the security codes used in end-to-end encryption may change. This is likely because someone reinstalled WhatsApp or switched phones.” It does not appear to detail the automatic resending aspect of in-transit messages.

This re-encryption and rebroadcasting of previously undelivered messages could potentially allow a third party to intercept and read a user’s undelivered messages in a situation where, for example, they had stolen a user’s sim card. When the third party put the stolen sim card in another phone, they could then theoretically collect any messages that had not yet been delivered to the user in question. (See first endnote.)

Many security experts, however, say that the feature is a known and acceptable trade-off that makes sense for the majority of WhatsApp’s users, since it makes the app easier to use on a day-to-day basis.

They describe the risk to most users as remote. In essence the experts say the threat to security is small and unlikely and would be very difficult even for a sophisticated attacker, who would have other ways to target someone. Timing would be hard and available messages few. The trade-off only allows potential targeting of individuals, not mass surveillance.

The group of experts urge WhatsApp users not to switch to less secure platforms. They point out that WhatsApp’s popularity means some users may be safer in this big crowd. If they switched to a service which was more secure but had relatively few users it might mark them out as an activist.

A WhatsApp spokesperson told the Guardian: “Over one billion people use WhatsApp today because it is simple, fast, reliable and secure. At WhatsApp, we’ve always believed that people’s conversations should be secure and private. Last year, we gave all our users a better level of security by making every message, photo, video, file and call end-to-end encrypted by default. As we introduce features like end-to-end encryption, we focus on keeping the product simple and take into consideration how it’s used every day around the world.

“In WhatsApp’s implementation of the Signal protocol, we have a ‘Show Security Notifications’ setting (option under Settings > Account > Security) that notifies you when a contact’s security code has changed. We know the most common reasons this happens are because someone has switched phones or reinstalled WhatsApp. This is because in many parts of the world, people frequently change devices and sim cards. In these situations, we want to make sure people’s messages are delivered, not lost in transit.”

Asked to comment specifically on whether Facebook/WhatApp had accessed users’ messages and whether it had done so at the request of government agencies or other third parties, the company directed the Guardian to its site that details aggregate data on government requests by country.

WhatsApp added later: “WhatsApp does not give governments a ‘backdoor’ into its systems and would fight any government request to create a backdoor.”

Concerns over the privacy of WhatsApp users have been repeatedly highlighted since Facebook acquired the company for $22bn in 2014. In August 2015, Facebook announced a change to the privacy policy governing WhatsApp that allowed the social network to merge data from WhatsApp users and Facebook, including phone numbers and app usage, for advertising and development purposes.

Facebook halted the use of the shared user data for advertising purposes in November after pressure from the pan-European data protection agency group Article 29 Working Party in October. The European commission then filed charges against Facebook for providing “misleading” information in the run-up to the social network’s acquisition of WhatsApp, following its data-sharing change.

• Facebook introduced two-step verification to greatly improve WhatsApp message security in February 2017, after testing it from November 2016.

• This article was amended on 13 January 2017 to remove the word “backdoor” to describe the design feature; on 25 January 2017 to summarise and link to concerns of security experts about the article; and on 28 June 2017 following the findings of a review by the Guardian readers’ editor, which can be read here.