Chief executive Sam Laidlaw dismissed the regulator's figures as "theoretical", as the energy firm's own data suggested profits from each household would be half that estimated by the regulator.
Energy companies are under intense political pressure over bills, with calls for falling wholesale gas and electricity prices to be reflected in customer's bills.
But half-year results from British Gas, owner of Scottish Gas, showed its operating profits dropped 26% to £265 million, with earnings hit by mild winter weather.
Latest figures from Ofgem suggest energy suppliers will make a pre-tax profit of £106 per household over the coming year, up from £53 last year.
But according to Centrica, British Gas will make only £40 per household in 2014, or £51 before tax, a 20 per cent fall on last year.
Mr Laidlaw said: "The Ofgem analysis is a theoretical analysis. What we are actually publishing today is the actual facts.
"We have been in discussions with Ofgem for a number of years about this methodology, which has its deficiencies and they recognise it needs to be changed."
British Gas customers are paying higher tariffs this year after they were hiked 9.2 per cent last autumn, though this was scaled back after the Government's shake-up of so-called green levies on bills.
But Centrica said the average bill was expected to be £90 lower this year, reflecting warmer weather and energy efficiency measures.
Separate figures from French-owned EDF - another of the Big Six - showed operating profits in the UK rose 9.5 per cent to £560m for the first half of the year boosted by its nuclear output.