The group said operating profit for the period was 26% lower than last year at £265 million at its UK residential energy supply arm as earnings were dented by the mild weather.
It said profit margins from the business would be lower for the year, despite new figures from regulator Ofgem suggesting that suppliers were making an increasing amount of money from the typical household, up to £106 for the coming year.
Adjusted operating profit for the wider Centrica group was also hit in the first half, falling 35% to £1.03 billion.
Chief executive Sam Laidlaw said: "With challenging trading conditions on both sides of the Atlantic in the first half, earnings will be lower in 2014 than in 2013. However, the group is well positioned to return to growth in 2015."
British Gas customers are paying higher tariffs this year after they were hiked 9.2% last autumn, though the rise was scaled back following the Government's shake-up of so-called green levies on bills.
But Centrica said the average bill was expected to be £90, or 7%, lower this year reflecting warmer weather and energy efficiency measures.
The company reiterated that tariffs were not expected to change during 2014 "recognising competitive conditions in the UK energy supply market".
It has come under pressure to cut bills after Ofgem pointed to falling wholesale gas and electricity prices - while it is also facing a full-scale competition probe which could see it vulnerable to a break-up.
However latest figures from the regulator suggesting rising pre-tax profit margins from household supply are disputed by industry body Energy UK.
Centrica defended its stance by pointing to the way it buys much of its energy in advance and that the benefit of lower wholesale prices for next year is offset by higher costs elsewhere.
It appeared to rebut Ofgem's claims about profits by saying post-tax margins for this year were expected to be about 4%, lower than last year and below the 4.5%-5% band it says it needs to support investment in the business.
Meanwhile, the company said British Gas customer account numbers had stabilised over the second quarter, after a 1% decline in the first quarter. They numbered 15.1 million at the end of June.
The supplier also launched a smart meter-based "free Saturday or Sundays" energy tariff trial that has previously been trialled.
In the US, Centrica's business saw earnings held back as it faced additional power market charges during harsh weather at the start of the year.
Chairman Rick Haythornthwaite said: "The first half of the year has seen challenging market conditions across the group, both as a result of the weather and reflecting the wider political environment."
He paid tribute to the "exceptional leadership" of Mr Laidlaw, who is to retire at the end of this year. It was announced earlier this week that he will be replaced by BP executive Iain Conn.