Reform UK’s showcase council in Kent faces “extreme risk” and “instability”, opposition politicians have warned, after it passed its first budget.

The party’s councillors voted it through on Thursday night after their leaders announced a 3.99% council tax increase, one percentage point under the limit before a referendum is required, despite promising before their election win not to raise tax.

While it was described by the council leader, Linden Kemkaran, as a “sensible, low-tax budget,” opposition parties said it left the council’s reserves “dangerously low” and exposed to financial risk totalling more than £410m on the local authority’s risk register.

The vote was carried with 48 councillors in favour – the number who are still with Reform after a period of turmoil in which several left Nigel Farage’s bloc on the council – with 26 against and one abstention.

The plan was described as a “casino budget” by Antony Hook, the Liberal Democrat opposition leader, who brought along letters and leaflets delivered by Reform before last year’s local council elections which pledged to cut council tax.

“It contains [Kent county council’s] highest ever exposure to financial risk – £411m of immediate risk in this year (up 60% from last year) and almost all is rated ‘4 out of 5’ in likelihood,” he said.

Accusing Reform of mounting an “attack on the vulnerable”, Hook singled out cuts of £1.2m from fostering services and £700,000 from fostering for children with disabilities.

Funding for infrastructure was static and did not keep pace with inflation, he added. “So, in real terms, Reform are cutting the highways budget. More potholes, more road collapses and a crumbling network,” said Hook.

The Green party’s Stuart Heaver described the budget as “a victory of political posturing over the interests of residents”.

“This is a showcase council for Nigel Farage and it was obvious the key priority for them is cosmetics – looking good – not serving the interests of Kent,” he said.

“The statutory financial assurance report was alarming to say the least – their budget risks the stability of the entire council. That doesn’t matter if they can boast that council tax was raised less than their predecessors.”

During the debate, the council’s deputy leader, Brian Collins, described the budget as being “carefully thought through”, adding: “Some people call it risky, I call it bold.”

Kemkaran had said Reform would prefer not to propose a council tax increase at all but given “the dire legacy we inherited”, alongside an unprecedented rise in demands and costs for services such as social care, “this is simply not possible”.

Budget documents showed that adult social care and health was “one of the most critical threats to the council’s financial resilience” with an estimated annual £68m risk.