ScottishPower is to cut its gas prices by 4.8% from 20 February, a week before the British Gas 5% cut kicks in.
The £33 annual reduction in the Scottish Power standard gas tariff for customers paying monthly by direct debit (and the £37 fall in British Gas's) follow the announcement of E.on's 3.5% (£24) reduction on 13 January.
While the ScottishPower and British Gas cuts are bigger, their customers have to wait much longer to see the benefit than E.On's, who saw their tariff slashed immediately.
Neil Clitheroe, ScottishPower's chief executive of retail and generation, said customers already on fixed-priced tariffs can also move between the company's tariffs at any time, without paying any exit fees.
Secretary of State for Energy and Climate Change 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."
Stephen Murray, energy expert at comparison site MoneySuperMarket, added: "Scottish Power's announcement today marks the half-way point of cuts from the Big Six energy suppliers. Unfortunately, again it feels half-hearted when set against a recent 20% cut in wholesale gas price."
He said the ScottishPower saving which sees the standard tariff average annual bill fall from £1,188.62 to £1,155.62 "pales in comparison to the huge bill cut possible if you take the bull by the horns and become your own energy boss".
Murray also pointed out that Scottish Power's prices remains far higher than competitors. The average monthly direct debit customer can benefit from savings of up to £256 by switching to Extra Energy's Fresh Fixed Price Jan 2016 v8.1, he added.