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Two more energy suppliers, E.On and British Gas, are to freeze their standard tariffs for the coming winter.

The move, for both gas and electricity bills, will affect around nine million customers in total.

Both suppliers have promised that standard tariff customers will now see no increase in charges until at least April 2017.

The decision follows similar action from SSE, which said it would cap standard tariffs for the same period.

It comes amid mounting political pressure on the big suppliers to treat loyal customers more fairly.

British Gas's announcement was made in a series of adverts in national newspapers. It said its standard tariff would "remain unchanged through the winter", which is understood to mean at least until the end of March 2017.

Consumer group Which? welcomed the announcement, but said standard tariffs were the most expensive on the market, and customers should consider switching from them.

"Energy companies should be doing much more than simply freezing their prices this winter," said Alex Neill, the managing director of home and legal services at Which?.

"We want them to set out what they're doing to genuinely engage with customers who are stuck on these poor-value deals."

However, both E.On and British Gas promised to do all they could to make customers aware of the best deals.

"We will continue to engage our customers on a wide range of topics including making them aware of their different tariff choices, helping them to use no more energy than they need and getting Smart meters into homes across Britain," E.On said.

The changes appear designed to counter criticism that big energy firms are taking advantage of customers that do not switch supplier.

Last month, Business Secretary Greg Clark met industry representatives amid claims that energy suppliers were profiteering from loyal customers.

In a statement at the time, he said that "customers who are loyal to their energy supplier should be treated well, not taken for a ride, and it's high time the big companies recognised this".

"I have made clear to the big firms that this can't go on, and they must treat customers properly or be made to do so."

A number of small energy suppliers have increased prices in recent weeks following rising wholesale prices.

But industry experts believe that the big six are unlikely to increase standard tariffs as they will have already bought gas and electricity for delivery in the coming months at lower prices.

Consumer groups have also highlighted the big difference in price between standard tariffs and some of the cheapest fixed price deals on the market.

According to price comparison site uSwitch, the British Gas standard tariff is the lowest of the big six at an average of £1,044 a year, but it is still £182 more expensive than the best deal on the market.

British Gas says the differential between its Standard Tariff and its cheapest deal is now just £40 a year.