Bar staff turned up for work to discover the locks on their pub had been changed and the business closed down.
Katharine Hammond, the tenant and general manager of Hanrahan's at Brighton Marina for five years, said X-Leisure, which owns the building and the marina complex, shut it down overnight, putting her and 20 staff out of work without warning.
She later discovered the lease for the building had been advertised for sale without her knowledge, with a note on the estate agent's web site saying: "Confidentially available - tenant unaware, upon instructions of X-Leisure."
Ms Hammond, 44, was abroad on holiday at the beginning of last month when she received a call from her cleaners telling her they could not get into the pub.
She contacted her manager, Craig Herbert, who arrived at the pub to find the locks changed and a sign up saying X-Leisure had retained the building.
X-Leisure says it was forced to terminate the lease because Ms Hammond was not paying the full rent.
She claimed a discount agreed with a previous owner should have continued after X-Leisure took over last year but the issue had not been resolved.
Ms Hammond said: "I came back from holiday early to find the place shut down. I couldn't get an explanation from anyone.
"It is no way to treat me and 20 other staff who have worked loyally for years. If it was about the rent then why did they wait a year and why did they not follow proper legal procedure?
"If someone is defaulting on rent - which I wasn't - then you pursue them through the courts, which they never did. They also did not seize any of my goods.
"I had more than £60,000 of goods inside the building which I was allowed to collect two weeks later."
Ms Hammond lived in a flat opposite the pub paid for by X-Leisure but has since moved to Corby, Northamptonshire.
She said she had spent more than £30,000 refurbishing the pub.
She owned the lease to the building under previous Marina owners Partridge's and claimed X-Leisure had repeatedly promised her she would be offered a renewal.
She said: "I spent years building up that business in the belief I was going to get a lease."
Instead, the lease was advertised on Oakley Commercial's web site.
Andy Hillion, 25, of Varndean Road, Brighton, was assistant manager at Hanrahan's for a year before the closure.
He said: "I got a call from Craig saying the locks had been changed and no one could get in.
"I never received anything from X-Leisure telling me my employment was ending or giving any explanation for what they have done.
"I have also paid for a management training course which is now in jeopardy as it is work-based and you need to be in a management position to complete it.
"We are all disappointed at the way we have been treated."
A spokeswoman for X-Leisure said: "The owners granted a Tenancy at Will, which was terminable by either side at no notice.
"Unfortunately, Mrs Hammond had refused to pay the full rent under the tenancy agreement, which left X-Leisure no alternative but to terminate."
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