Scottish Gas owner Centrica yesterday warned customers to prepare for a second major rise in energy bills, with analysts predicting increases of more than 15%.

Britain's biggest energy supplier said that profit margins in the first half of the year had been "squeezed to levels below our long-run expectations" and operating profit for the first half of the year would be "materially lower" than in 2007.

It expects wholesale gas and electricity prices to remain "stubbornly high" for the next six months and signalled it may have to raise electricity and gas prices further to cope with increasing costs.

Centrica said: "While the current outlook for gas prices does create a challenging environment for energy suppliers, we will take the necessary action to deliver reasonable margins in the retail business."

All major energy suppliers have increased their energy tariffs so far this year, with British Gas, which includes Scottish Gas, announcing a 15% average price increase for gas and electricity in January. It pushed an average annual bill for gas and electricity from £912 to £1051.

Peter Atherton, an analyst at Citi, said: "In our view this clearly flags that Centrica intends to raise retail prices again later this year. We expect Centrica to raise retail prices by 12% to 18%."

Online comparison service uSwitch.com said that the outlook for customers of all energy suppliers was "bleak". It predicted an average 10% price rise by late summer, with a further 15% hike in January 2009. It will mean the average dual fuel bill will rise from £912 at the start of the year to £1327 by January 2009 - a 46% increase in 12 months. Tim Wolfenden, head of home services at uSwitch.com, said: "If Centrica - the parent of Britain's biggest supplier - is feeling such acute pressure over pricing then it's safe to say that others are feeling it too.

"Suppliers have been holding firm, but the cracks are beginning to show. It's pretty clear that something has to give and that household energy prices are going to be shooting up again this year.

"Suppliers usually give consumers breathing space by introducing a couple of smaller price increases rather than hitting households in one fell swoop. The pressure they are under shouldn't be enough to change this pattern, but it could be enough to force their hand sooner."

Mr Wolfenden said that fixed-rate energy plans could be "a potential lifeline" for customers facing further price rises. Their tariffs are more expensive than on standard plans, but he said they offered "security at a time of rising prices, especially for those struggling with household bills or on a tight budget".

Centrica said that record oil prices, rising demand in Asia for liquefied natural gas and insufficient supply from continental Europe meant that prices had continued to rise for suppliers. Wholesale gas prices for the second half of this year are double that of the same period last year, it said.

Centrica's group operating profits for the first half last year were £1.25bn, with £1.95bn achieved for the full year.