Businesses could lose up to five years' worth of refunds on their business rates if they don't lodge an appeal before 1 April this year.
That's the warning from Colliers International, which warned that the government had hidden the deadline in the Chancellor's Autumn Statement just weeks ago, in December 2014.
John Webber, head of rating at Colliers International, said he was concerned that most businesses were unaware of the government-imposed deadline, which was "almost lost in the small print" of the Autumn Statement.
"Despite having extended the time period for the revaluation of businesses on the ratings list from five to seven years (extended to 2017) they haven't extended the period where businesses are entitled to appeal," Webber said.
"This means that while businesses will still be able to appeal between April 2015 and 2017, any refunds or savings will only be backdated to April 2015, meaning that businesses could lose out on five years' worth of rebates.
The government has not explained its move but Webber said it was designed to curtail the ability of businesses to appeal while giving it more certainty about what it collects in tax.
He said: "Most businesses won't be aware of these changes. They haven't been publicised and there are genuine concerns that less business rate savvy companies will really lose out.
"We worry that the new deadline will cause panic with 'cowboys' in the industry persuading clients to appeal en-masse to the Valuation Office without the due diligence which could result in an increase in rates liability - ironically, also dating back five years."
Colliers International has set up a hotline for businesses to call, to find out more about this issue and to find out whether they are entitled to a rebate: 0800 3583230