Ryanair has called on the Office of Fair Trading (OFT) to investigate price rises in aviation fuel at Belfast City and Glasgow Prestwick airports.
The budget airline wants a probe into Air BP's "unjustified attempts to impose monopoly increases of over 50%" on the delivery of fuel.
Ryanair said Air BP, which has a "complete monopoly" on the fuel at the airports, has not explained the rise.
Air BP, which provides air services and fuels, said the prices were reasonable.
The firm said prices are agreed "by mutual negotiation", and must "reflect current market and cost environments".
It said it was "confident" that it had not breached any competition laws and was surprised by the carrier's decision to involve the OFT.
Meanwhile Ryanair said it has repeatedly written to the fuel provider asking for an explanation for "these rapacious increases" but claimed Air BP had "refused to provide any valid justification".
"Air BP has a complete monopoly on the supply of aviation fuel at these airports and is abusing this position to unilaterally impose over 50% cost increases (over 10 times the rate of inflation)," said Jim Callaghan, Ryanair's director of legal and regulatory affairs.
"This abuse comes at a time when BP and the other oil majors are making billion pound profits at the expense of the aviation industry, which is currently in crisis," he added.
A number airlines have folded as they face the twin challenge of rising costs of fuel and lower consumer demand.
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