All the signs are that even after these punitive price increases, the poorest consumers are less likely or less able to switch supplier

Audrey Gallacher, energywatch

Energywatch Scotland, the independent watchdog for gas and electricity consumers, said the expected rise was "wholly unjustified" given stable wholesale gas prices.

It accused ScottishPower of blatant "opportunism" so shortly after Scottish Gas had increased its prices.

Watchdog director Audrey Gallacher said: "Research commissioned by energywatch in the wake of the Scottish Gas increase indicated that customers are set to desert Scottish Gas in droves.

"It is highly likely that ScottishPower will suffer a similar fate as gas consumers follow our advice and shop around for a better deal. Energywatch Scotland is once again arguing that the market is not working for the most vulnerable of energy consumers.

"Between them ScottishPower and Scottish Gas have a market share of around 90% of the Scottish market.

"All the signs are that even after these punitive price increases, the poorest consumers are less likely or less able to switch supplier.

"This can be for a number of reasons, such as being blocked from switching because they are in debt to their current supplier.

"Whatever the reason, these price increases will cause the most hardship to the poorest in a sector where competition is almost non-existent."