The party won 33 seats on Thanet District Council, Kent, in May, making it the first authority in the UK to have a UKIP majority.
UKIP councillor Jeff Elenor announced on Tuesday he was defecting to join the Democratic Independent Group.
He joins four other UKIP councillors who defected last month saying they were unhappy with the council's lack of action over Manston Airport.
They also said they were unhappy about what they felt was insufficient focus by the council on issues such as housing, mental health and disabilities.
Mr Elenor said he did not feel the council was fulfilling its election pledge on Manston.
In May, UKIP won 33 seats, the Conservatives 18, Labour four, and independents one - 29 seats were needed for a majority.
UKIP now has 28 members, but remains the largest party on the council.
Earlier this week, the US company seeking to purchase and reopen Manston Airport accused the council leader of holding up the process.
The airport closed in May 2014 and was sold to a private consortium that plans to build homes on the site.
RiverOak said it had offered the money to help with a compulsory purchase order (CPO), and in an email to the council leader, it accused Chris Wells of "throwing one road block after another in front of us in order for you to carry out whatever your real agenda might be".
"Whatever it is, that agenda does not appear to recognise the will of your constituents, which is that Manston Airport be opened and operating at the earliest possible moment," chief investment officer George Yerall said.
Mr Wells said the council was seeking further assurances that RiverOak could afford to purchase the site.
The matter is due to be formally discussed at a cabinet meeting later this month.
The UK Independence Party has lost overall control of its only council.