This increase represents a 40 per cent rise from the £1.80 previously indicated on contracts, despite new rules from telecoms regulator Ofcom.
Money expert Martin Lewis slammed O2's decision, stating it makes a "mockery" of the new guidelines and advised customers to switch providers penalty-free.
Ofcom expressed disappointment with O2's move, stating it "goes against the spirit" of rules designed to prevent surprise mid-contract price increases.
While O2 maintains its actions are within regulatory rules and customers can exit without penalty, Ofcom has reminded all major mobile companies of their obligations to treat customers fairly.