The government has asked the media regulator to revisit its rules on phone companies raising their prices in the middle of a contract, after O2 unexpectedly announced it was raising prices by £2.50 a month.
Technology Secretary Liz Kendall said O2's higher than expected price increase is "disappointing given the current pressures on consumers".
"I believe we need to go further, faster. I am keen that we look at in-contract price rises again," she wrote in a letter to the media regulator.
Ofcom said it shared the government's concern "customers who face price rises must be treated fairly by mobile providers".
O2 said in a statement: "We appreciate that price changes are never welcome, but we have been fully transparent with our customers about this change, writing directly to them and providing the right to exit without penalty if they wish."
