Scottish Power is to cut domestic gas prices by 4.8%, the Big Six energy supplier announced today.

It said the cut will see the average annual gas bill on a standard tariff reduced by £33. It comes a day after a 5% cut was announced by British Gas.

Energy firms have been under pressure from politicians and regulators to pass on the fall in wholesale gas prices to consumers. E.ON last week cut gas prices by 3.5%.

The latest announcement will affect 1.1 million customers who are on standard or variable rates but not those on fixed tariffs.

The firm, owned by Spain's Iberdrola, also announced a new fixed-price deal which it said was one of the most competitive in the marketplace with an average dual-fuel bill of £930.

The gas price cut means that half of the Big Six firms that dominate household gas and energy supply in the UK have now reduced tariffs this month following sharp falls in the cost of energy on the wholesale market.

It will pile pressure on rivals SSE, npower and EDF to do the same.

Neil Clitheroe, Scottish Power's head of retail and generation, said: "Today's decision has been made to benefit our customers and keep our prices competitive.

"We will continue to keep our prices under review. Our pricing reflects all of the costs that contribute to a customer's bill.

"The wholesale price of energy accounts for half of a customer's gas bill, but non-energy costs such as transmission and distribution networks and environmental and social obligations remain unaffected by any wholesale energy price movements."

Customers already on fixed tariffs can move between tariffs at any time without paying exit fees, he added.

The gas price cuts have thrown the energy sector back into the spotlight four months ahead of the general election in which Labour will pledge to cap tariffs.

Prime Minister David Cameron said in response to yesterday's British Gas cut that it would not have been possible under a price freeze by Labour.

The reductions have fallen short of industry estimates suggesting bills could fall by £136 a year if suppliers were to pass on in full the drop in wholesale prices.

Industry experts believe the firms may be reluctant to make deeper cuts for fear they will be locked into them should Labour win the election and cap prices.

Scottish Power's announcement comes two weeks after Chancellor George Osborne launched an investigation into whether key sectors such as energy firms were passing on the costs of falling wholesale prices to consumers.

Labour proposed giving Ofgem new powers to force suppliers to do so.

The sector is also in the midst of an in-depth investigation by the Competition and Markets Authority which could result in bigger players such as British Gas being broken up.

Energy Secretary Ed Davey said: "Competition is hotting up and customers will see the benefits. With the market getting more competitive there's even more pressure on other energy companies to drop their prices.

"If people aren't seeing price cuts, now is the perfect time to check their tariff and see what deals are on the market. Switching suppliers is quicker and easier than ever before as part of our work to cut energy bills and reform the market."

Martin Lewis, founder of independent consumer website moneysavingexpert.com, said: "With the wholesale price of energy having come down by about 20%-30% since the start of 2014, and the prices of the cheapest switchers' deals having dropped over 10%, these cuts are trivial.

"I expect the remaining three of the big six will rapidly follow now, shaving down their gas prices slightly too - but nowhere near the amount we should've seen after the wholesale price cuts."

Shadow energy secretary Caroline Flint said: "This latest price cut shows that Ed Miliband was right to challenge the energy companies to pass on the falls in wholesale costs to consumers.

"But given wholesale gas prices have fallen by at least 20%, a price cut of just 4.8% means consumers are only seeing a fraction of the savings.

"This price cut has also been delayed for a month, meaning consumers won't see any benefit at all until winter is almost over.

"The next Labour government is committed to making big changes in our energy market: freezing energy prices until 2017 so that bills can fall but not rise, and giving the regulator the power to force energy companies to cut their prices and pass on the full savings from wholesale cost falls to all consumers."

James Padmore, head of energy at price comparison website comparethemarket.com, said the other Big Six energy suppliers were likely to follow in a "domino effect" of falling gas prices.

He said: "Expect further pressure on the size of the price cuts - an annual saving of £33 is tiny compared to that offered by one of the smaller suppliers, with savings up to £400.

"High-profile political price-freeze pledges - while well intentioned - could be holding back the savings that the Big Six are able to pass on, as they do not want to be locked into lower deals, especially if the wholesale price of gas rises."

A spokesman for SSE said: "We're continually looking at our prices and costs and when we announced our unprecedented price freeze to 2016 we said we'd cut prices if we can.

"Energy companies like SSE buy up to two years in advance to protect customers from volatility in wholesale prices but, rest assured, if we can lower prices or extend our freeze, we will."