The tribunal found that in November 2019, a site manager at house builder Crest Nicholson had harassed the woman and another female staff member during the work event, tried to kiss her in a taxi and then raped her in a hotel room.
He then contacted the woman in December 2019 to persuade her not to tell anyone about the rape, saying “both our families have a lot to lose".
Neither the claimant nor the site manager were named in the case.
Their employer, Crest Nicholson, argued it should not be at fault for the actions of the site manager because they took place outside the workplace.
However, the tribunal judge said the site manager's misconduct happened in the course of his employment and found the employer liable for both the initial sexual misconduct and the threatening message a month later.
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Michelle Last, employment partner at Keystone Law said employers should be aware of liability for misconduct at work events as they are often interpreted as an extension of the workplace.
Speaking to HR magazine, she said: “Employees who are sexually harassed and/or assaulted at or following a work event can successfully claim their employer is liable for the acts of their harasser.
“Simply because the event takes place outside working hours is not sufficient to break the connection.”
In this case, Last added, the fact the site manager was found to have harassed two women in the event and that the event was paid for by the employer strengthened the connection of liability.
“In particular, where the alleged harasser has a track record of sexually harassing and/or assaulting other employees, whether at, before or after the event, there is an increased risk the employer will be held liable for the acts of the harasser.
“In addition, where the employer is paying the costs for staff to attend, this also points to the event being deemed to be in the course of employment.
“Tribunals take a common-sense approach to such situations and if an act is connected to the employment relationship, employers may face liability.”
Kate Palmer, HR advice and consultancy director at business services provider Peninsula, said employers should treat work events as an extension of the workplace.
Speaking to HR magazine, she said: “An employer’s duty to protect their staff from harm, and act where necessary to fulfil this duty, doesn’t stop the moment an employee finishes their working day and leaves the building. A classic example of this is the Christmas party.
"Sadly, for the woman in this case, her employer failed in that duty, missing several opportunities to protect her and her colleagues from harm.”
Palmer said protecting employees starts with creating a respectful culture.
“Setting behavioural expectations via company policies, reminding employees of the need to act appropriately and with respect to their colleagues, and to drink responsibly, are all important parts of discharging this duty towards employees,” she said.
In May, The Chartered Management Institute issued a warning to businesses to reduce the amount of alcohol served at workplace social gatherings.
The warning came after a poll of 1,000 managers found 29% had witnessed harassment or inappropriate behaviour at office parties.
Palmer said events should not be centred around alcohol both to reduce inappropriate behaviour and to include those who cannot drink alcohol.
She said: “Finding alternative ways to celebrate with employees, such as activity days or providing food and a chance to socialise with colleagues, can also be enjoyable and come with a much-reduced risk than an afternoon with a free bar and drinking games.”
