The government's delays in implementing a new scheme of European farm payments could end up costing the taxpayer at least £20 million, the Tories say.
Farming subsidy changes were made by the EU last year
Conservative agriculture spokesman Jim Paice said this was unacceptable as the government had been told last year that the system was heading for disaster.
Under EU rules, member countries must have made payments to farmers by the end of June, or face heavy fines.
The government has already said it might miss the deadline.
It had asked the European Union for an extension, but sources said this would amount to rewarding the UK for not making the system work.
The subsidy system was changed last year - providing that instead of being paid for the food they produced, farmers would be paid for the environmental management of their land.
Farmers' leaders say the delays and mistakes in the way the system has been implemented in Britain are a "tragedy" and could lead to some bankruptcies.
Mr Paice said that even if Britain manages to pay the subsidies by the end of July, it is still facing a fine of £20m.
The fine rises substantially for every month payment is overdue.
Earlier, Environment Secretary Margaret Beckett acknowledged a "human crisis" caused by the delays, but said "partial payments" of 80% of the amount claimed would now be paid.