The way markets are structured is critical to delivering an efficient allocation of resources. During the 1980's the movement to increase competition in markets in order to drive prices down and consumer service up was the basis of the privatisation and deregulation programme.
The idea is essentially that monopolies can exploit their customers and the more firms in the market the better it is for keeping costs and prices down.
In some markets economies of scale means you can't efficiently have hundreds of firms. One such market is energy supply, but you could certainly have up to 15 or 20. But when you have oligopoly market structures there is a tendency to collude and so some regulation to ensure fairness and maintain competition is needed.
So the energy market, dominated by six big suppliers, is one that might fall into a situation where they each decide not to compete too hard for business and all keep prices quite high.
Many people have been disappointed by recent high energy prices, some believe that the firms are keeping prices high when they need not. Others think that pricing is unfair with some people getting good deals while others (without Internet or able to pay by direct debit) pay too much.
So the government are introducing a new regime where there will be fewer price plans and customers will have to be put on the lowest available tariff.
This seems to be abandoning the idea of competition as a way of making sure resources are allocated efficiently.
The fear is that this will eliminate competition. The big firms will set similar but higher prices and while some people will be better off the good deals will disappear. The danger is that the government has just done the firms collusion for them and will prevent prices being forced down in the market.