Calls for council tax to be cut by Merton Conservatives have been slammed by councillors from other parties.
Councillors will vote on a Labour motion to freeze council tax at a special budget meeting tomorrow.
While they may have expected a smooth vote, Merton Tories unhinged plans last week with their proposal to go further and cut tax by 2.5 per cent.
Now the council and other opposition parties have hit back at the Tories' proposed amendment to use council reserves to subsidise a tax cut, suggesting their finance calculations don't add up.
Councillor Mark Allison, cabinet member for finance, said: "The difficulty with using reserves is if this is a 2.5 per cent cut and you want to use reserves you can cut the tax for one year but you will have to raise it for next year.
"I think they are doing it because they don't know how finances work.
"They have returned to the days of uncertainty and council tax bouncing around and people not knowing what to do.
"You need to have money for a rainy day. With budget cuts anticipated in future years it would be irresponsible to have no money in reserves.
"There's no way the Conservatives can cut council tax consistently without cutting services."
The Labour-led council said if it is re-elected in May there will be no additional cuts to services in the next 12 months, because they started planning the budget in 2011.
Councillor Iain Dysart, leader of Merton Liberal Democrats, said: "We're pleased that council tax will once again be frozen because the coalition government provided extra cash to do so.
"But the local Conservatives' plans to cut council tax by 2.5 per cent look like they were drawn up on the back of an envelope.
"When they last ran Merton Council they put council tax up every year, and so their plans now just don't seem believable.
"To pass their amendment would mean more cuts to local services, and they know that the other groups will have to vote against - they're just playing politics because there's an election coming."
The government has offered a grant equivalent to a 1% rise in council tax to local authorities who freeze council tax.
Coun Allison has calculated this would add £850,000 to council reserves.
UKIP councillors are supporting the Tory pledge to cut council tax by drawing funds from the council's general reserves.
Councillor Suzanne Evans, leader of UKIP, said: "I can now confirm UKIP Councillors will be calling for a £70 Council Tax rebate for 75,000 households in Merton on Wednesday.
"The ruling Labour party stood for election in 2010 on a manifesto pledge of giving a £125 rebate over four years, so I see no reason why they shouldn't vote now for this rebate now, which is easily affordable and can be given without doing any harm to Council finances."
Cabinet agreed to freeze council tax at a meeting last week, but the decision needs to be ratified at full council on Wednesday.