An average annual household bill of £300 could increase by between £9 and £28, the Energy Contract Company has warned.
The company said high continental wholesale gas prices and the possibility of war in Iraq were pushing up prices.
Gas prices are linked to the price of oil in much of Europe, meaning that recent rises in the oil price will push wholesale gas prices significantly higher.
Three of the UK's largest gas suppliers have already increased their prices this year.
Rising bills
British Gas' latest increase means an average rise of 2.5% on its 13m customers' domestic bills.
Consumer group Energywatch has written to BG asking it to explain the price rises.
"We are totally at a loss as to why they would be putting up their prices.
"This comes at a time when its profits from energy residential sale have risen by 1000%," a spokeswoman told BBC News Online.
But the biggest single price rise so far this year will be coming from Powergen TXU.
The company announced on 11 February that its Powergen customers would see their bills rise by on average 6.2% or the equivalent of £18 on a £300-a-year bill household bill from 1 May.
Customers of TXU will see their bills increase by slightly less at 3.45% on average.
Npower is another supplier who is putting up its bills.
From 1 April, prices will go up by on average 4.3%.
The move will add £13 to a £300 household bill.
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