The price cap is designed to help those customers who have the highest monthly bills

The government’s energy price cap is in danger of making bills more expensive after companies increased the prices of their cheapest deals by 21 per cent in five months.

Analysis of the 30 best annual dual-fuel tariffs on the market found that they were on average £178 more expensive than at the same time in May, rising from £864 to £1,042.

The comparison site Moneysupermarket said that many companies were likely to be getting as close as they could to £1,136, which is the level of the first mandatory energy price cap, due to be introduced in December. “The wholesale market [where gas and electricity prices have increased] is one big driver of this trend, but the energy price cap is another factor,” Stephen Murray,