The occupation, which involves around 40 people, followed a student and staff-led demonstration on 7 February protesting against outsourcing at the institution.
Estates and catering workers are among the 235 staff members who will be transferred to private companies from August 2013.
In a statement the group said that the occupation had come as “a last recourse for action in order to ensure that student and staff voices are heard”.
At the rally, protesters were asked to come with yellow items of clothing, while staff showing solidarity with the workers have this week been sticking yellow squares of paper in their office windows. A further demonstration is planned for today.
The university is currently in a period of “competitive dialogue” with potential companies for the outsourced contracts, with firms to be identified in spring this year.
Although under TUPE employment regulations staff would be transferred to the companies on their current terms and conditions, protesters against the outsourcing claim in the long term staff risk worse wages, job security and redundancies. A statement from the university says no redundancies are proposed by the change.
In a letter to the university’s vice-chancellor, Michael Farthing, procurement specialist and former national secretary at Unison, Malcolm Wing, calls the process at Sussex “without doubt the worst in terms of engagement with staff and their trade unions and the most secretive” in his experience.
The decision last year to communicate the process to affected staff on the same day that a contract notice was published was “unprecedented” and among the staff there remained a “great deal of concern and anxiety about the future”, the Sussex alumnus adds in the letter emailed on 6 February.
University of Sussex registrar John Duffy, who is responsible for the university’s professional services, said the outsourcing was to ensure the university could “enhance and develop” services as the institution grows from fewer than 11,000 students in 2008 to a projected 18,000 by 2018.
A spokeswoman for the university said that the workers who were set to transfer to the companies, the three affected unions and other staff and students were being kept informed about the process.
Next week will see a question and answer session for students and two open meetings with the vice-chancellor on a range of campus developments, she added.
The students’ union said it feared that by following the outsourcing proposals the university would be “putting the student experience in serious threat with something which has been acknowledged as a high-risk move”.
Caroline Lucas, Green MP for Brighton Pavilion, issued a statement lending her full support to the protesting staff and students.
“It is deeply concerning that the decision to outsource university services has been made with so little transparency or consultation - making it impossible to know whether these plans are either sustainable or good value for money,” she said.
“I’ve raised these fears with the management and am still waiting for an answer as to why it refused to consider any in-house service improvement plan, instead presenting ‘doing nothing’ as the only alternative to outsourcing.”