The UK government has incurred fines of £642m from the European Commission for the way it manages EU farming payments.
The National Audit Office (NAO) said the penalties, imposed since 2005, gave the UK the sixth worst record of the 28 EU member states.
It said the government had made progress in the past year in managing the Common Agricultural Policy fines.
But it warned the amount of fines levied was likely to rise because the policy had been made more complex.
The Common Agricultural Policy (CAP), which supports the agricultural sector with subsidies, is the most expensive scheme in the EU, accounting for about 38% of its budget.
The money is spent on direct payments to farmers and rural development programmes.
The European Commission can impose fines where it does not think member states have complied with its guidelines.
The NAO said that of every £100 received from Brussels through the policy, £2.70 was given back in fines, with late payments to farmers and poor mapping data among the reasons.
Despite attempts to make the scheme more simple, the NAO said the updated policy had become more complex - with the government warning it will be 15% more expensive to manage than the old version.
Meg Hillier, the chairwoman of the Commons Public Accounts Committee, which oversees the work of the NAO, said the prospect of further fines was "frightening".
"Complicated CAP rules don't help but the department must get a grip to deal with these persistent problems and ensure we aren't throwing away taxpayers' money on financial penalties," she said.
The Department for Environment, Food and Rural Affairs is considering spending £25m to £45m to improve mapping and avoid penalties of up to £370m, the report said.