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.

Norma Diffley, who ran a boutique in Donaghadee, died in the accident on the A2 dual carriageway between Bangor and Holywood close to the Devil's Elbow at Carney Hill on Monday afternoon.
The 33-year-old man is being questioned on suspicion of dangerous driving causing death.
He was arrested on Tuesday.

It came as a surge in commodity prices saw Brent crude oil rise above $40 a barrel for the first time this year.
The Dow Jones closed up 67.18 points at 17,073.95, while the broader S&P 500 rose 1.77 points to 2,001.76.
However, the tech-heavy Nasdaq index fared less well, falling 8.77 points to 4,708.25.
Oil firms led the Dow higher, with Chevron adding 3.1% and Exxon gaining 2.6%.

Thai officials said the event, which was halted minutes before it was due to start, could have affected relations between the two countries.
The HRW report focuses on the treatment of a Christian group in Vietnam.
The group said the Thai response showed how freedom of speech had been eroded since the army seized power last year.
Thai police said the event at the Foreign Correspondents Club of Thailand could "have an impact on the country's security or could affect the friendship and cooperation between Thailand and Vietnam".
It is the third human rights event at the venue that has been halted by authorities in the past month.
The HRW report describes what it says is the persecution of Montagnard Christians in Vietnam's central highlands. Their religious practices have been described by the Vietnamese government as "evil".
Sunai Phasuk, Human Rights Watch's senior researcher in Asia, said the decision to cancel the report's launch was "very disappointing".
"Thailand is now going to be known as the defender of human rights violators in [Southeast Asia], which adds more damage to Thailand's already tarnished international reputation under the military rule," he added.
Thai authorities have launched a crackdown on critics since the military seized power from a civilian government in May 2014.
The UK Independence Party has lost overall control of its only council.
[NEXT_CONCEPT]
A 33-year-old man has been arrested in connection with a two-vehicle crash which claimed the life of a 75-year-old woman in County Down.
[NEXT_CONCEPT]
(Close): Wall Street's Dow Jones and S&P 500 indexes closed higher for the fifth day running, in their longest winning streak since last October.
[NEXT_CONCEPT]
Thailand's military-run government has forced Human Rights Watch (HRW) to cancel an event in Bangkok to launch a report into alleged abuses in Vietnam.