Derogatory comments or jokes about a protected characteristic, such as commenting on someone’s age, sex, sexual orientation, gender reassignment, religious beliefs, disability, could amount to discrimination under section 13 or harassment under section 26 of the Equality Act 2010.

Dawn Dickson, a partner at Anderson Strathern, said jokes about someone’s appearance were “big risks” because they could come across as offensive.

“It doesn’t matter what the sender intended – it’s all about how the message is perceived and what effect it had,” she said. “Another concern is bullying – for example, when someone uses WhatsApp to mistreat a co-worker or leaves them out of a work group.

“If this kind of behaviour continues unchecked, it could lead to harassment claims or even constructive dismissal, which can be expensive for employers.”

Mark Brosnan was awarded more than £130,000 ($253,000) in compensation after he was excluded from a work WhatsApp group chat while on sick leave. The plumber argued he had been victimised by his employer setting up a group chat without him.

An employee criticising an employer or manager could also find their messages being used as evidence in an employment tribunal.

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Debbie Barker was dismissed from her role as night manager at the Corporation nightclub in Sheffield after describing her boss Mark Hobson as a “c---” and writing that he “can suck my b----” in a WhatsApp group called Rebel Alliance.

Barker later told the company: “We all trusted each other and felt safe within [the chat]. That was the level of trust we had in that our information was safe within the group and not to be shared outside.”

In another case, Mahnaz Rezvani, an HR executive, was made redundant by AHRO Scientific Publishing, in Glasgow, while pregnant, and was later sent a line of laughing emojis by her boss, Dr Abubakar Yaro.

Emma Nunn, an account manager, accused her Kibworth vehicle recovery company boss of sexual discrimination in April this year.

A tribunal heard she was asked by Adam Crouch to attend a meeting because a man attending liked “pretty women” before being called “Royder” – a nickname from “Emma Royd”, sounding like “haemorrhoid”.

In another tribunal, Georgina Roberts put in an unfair and wrongful dismissal and a sexual harassment claim against Goran Hankic, a recruitment director.

Roberts was sacked after rejecting the advances of her boss, who was on cocaine and attempted to climb into bed with her. In the month before the incident, the pair had shared flirtatious WhatsApp messages that would later be shown as evidence to the tribunal.

Mary Walker, a partner and employment law expert at Gordons, a law firm, said that “obscene or defamatory” messages must always be investigated by employers and could be considered grounds for dismissal.

She said: “Put simply, if it is something you wouldn’t say out loud, or to your family, it shouldn’t be on WhatsApp. Proactive steps from employers can address the informality of certain communications channels.”

The informality afforded by WhatsApp also carries the risk of work spilling over into private lives, with contact outside office hours.

When Hemanta Mainali received a late-night message from his colleague, he took it as a “deliberate attempt to disturb him” and reacted angrily. Mainali, a sushi kiosk boss, left the business soon afterwards and later sued his colleague for harassment.

Walker added: “In some cases, it is subjective as to what constitutes whether an inappropriate WhatsApp message is sackable. However, this highlights the importance of workplace culture.

“If one is fostered which dismisses obscene messages as “banter”, then the likelihood of a more toxic culture escalates and would be viewed poorly by outside audiences, or an employment tribunal.”