Portsmouth Water said it will also have a new tariff in place to help those on low incomes from July.
Wessex Water and Bristol Water already operate social tariffs but are both adding new schemes to help those who receive Pension Credit.
The Consumer Council for Water (CCWater) welcomed the new tariffs, which it said will “significantly reduce water bills for almost half a million customers who are struggling to pay”.
It did, however, warn that many households are missing out on the help available to them due to a lack of awareness or reluctance to ask for help.
CCWater senior policy manager Andy White said: “We know from our research that one in eight households feel their water bill is not affordable. Help is waiting for thousands of customers, but many simply don’t expect to be able to get assistance from their water company.”
The other water companies with social tariffs are Affinity Water, Anglian Water, Welsh Water, Essex and Suffolk Water, Northumbrian Water, Severn Trent Water, South East Water, South West Water, Southern Water, Sutton and East Surrey Water, Thames Water and Yorkshire Water.
In February, water regulator Ofwat released a report insisting water companies are not doing enough to help vulnerable customers, and must understand the individual circumstances of customers to offer more tailored support, as it was announced that the average water and sewerage bill will rise by £2 to £389 in financial year 2016/17.
It said although there has been a “big increase” in support for vulnerable customers, the sector still has a “long way to go” to meet customers’ needs.
Ofwat chief executive Cathryn Ross said at the time: “We know that there are many pressures on household budgets and so we challenged companies to provide much more support to customers who are struggling to make ends meet.
“Under those agreements around one million more people will be eligible to receive help. Some are making good progress in targeting support, while others need to get better at identifying those at risk and helping them. We will keep a close eye on how the sector progresses.”