Staff members at the People's Vote campaign have been threatened with dismissal if they do not return to work following the sacking of two of the group's key officials.
Forty members of staff signed a letter demanding director James McGrory and head of communications Tom Baldwin be reinstated.
The campaign's staff have not returned to work following a meeting with the organisation's chair, Roland Rudd, and chief executive, Patrick Heneghan, on Tuesday.
At the meeting, they said they received "no proper answers" to their questions about Mr McGrory and Mr Baldwin's dismissal and subsequently held a non-legally binding vote of no-confidence in both Mr Rudd and Mr Heneghan.
People Vote campaigns for another referendum on the UK's membership of the EU.
A source close to Mr Rudd said that if staff do not come into work, "then they will be replaced by staff who do want to work towards a People's Vote".
The source added: "Staff have been asked to return to work and we hope everyone who wishes to do so, will do so, without feeling intimidated by those who have orchestrated this walk out.
"The staffing decisions will not be reversed and new senior staff will be announced in due course."
In response, a member of staff told the BBC: "This is a coup against the people who built the People's Vote campaign by a multimillionaire who has contributed nothing to it."
Staff members have said they will return to work if Mr McGrory and Mr Baldwin are given their jobs back.
"We are all ready and willing to start work. We could be back at work if he [Mr Rudd] got out of the way," the staff member said.
The staff member added: "the reason we're all so passionate and angry is because this is such a massive week for the campaign."
They also told the BBC that while Mr Rudd and Mr Heneghan can "appoint who they want", they would not be able find 40 new campaigners with the same level of "experience and expertise".
Mr Rudd is chair of Open Britain, one of the five groups that makes up the People's Vote campaign, and which controls the contracts of most of those working for the organisation.
Another member of staff also described the events as a "coup" by people who "don't understand how political campaigns work".
Both members of staff criticised Mr Rudd's use of guards at the staff meeting on Tuesday, which took place at a hotel, with one describing it as an "intimidating atmosphere for just a meeting".
In response, the source close to Mr Rudd pointed out that, "Millbank, where they work every day, is guarded by security guards and surrounded by armed police".
The source also said that "no threats have been issued, just facts.
"The staff can turn up to work and be paid. Or not turn up to work and not be paid."
The campaign staff are now considering their next steps, and are "exploring every avenue" including potential legal options.
The chair of The European Movement UK, one of the groups that makes up the People's Vote campaign, has said he is "deeply concerned" with recent events and called for the organisation to be restructured.
In a letter to staff, Stephen Dorrell said: "Although our staff are not directly involved, and will continue to remain neutral, the associated publicity is deeply damaging to the campaign we have built together."
In the letter, Mr Dorrell goes on to call for "better governance, robust against recent failings" and that Open Britain should be owned jointly by the main organisations involved in the campaign.