Water companies are in the dock over their handling of supply disruptions during the so-called "Beast from the East" cold snaps in February and March.
A report by Ofwat found many households and businesses were badly let down - with just a few of the 17 firms across England and Wales being praised for their response.
The regulator said more than 200,000 customers in England and Wales were left without supplies for more than four hours while over 60,000 customers did not have a water supply for more than 12 hours.
Some were even without water for a week over the two bouts of freezing weather during February and March, Ofwat said.
It found that while engineering teams battled bravely to restore supplies, there were failures behind the scenes in areas such as planning, communication and co-ordination - including the provision of bottled water.
It found that many firms were hunting down emergency supplies at the same time and therefore bottled water suppliers struggled to meet demand.
The watchdog said four firms; Thames Water, Severn Trent, Southern Water and South East Water, had been given three months to provide a detailed, externally audited, action plan to tackle a range of concerns about their response.
Northumbrian Water, United Utilities, Wessex Water and Yorkshire Water were singled out for praise.
Separate research among customers affected by the Consumer Council for Water (CCW) suggested there was an over-reliance on Twitter to keep homes and businesses updated when frozen high pressure mains gave way - often when they started to thaw.
It found 40% of those who lost supplies received no communication from their provider at all.
Ofwat said water firms had paid out more than £7m in compensation to customers affected but demanded planning was improved to bolster customer service during future cold weather events.
Chief executive Rachel Fletcher said: "Too many companies were caught off guard and let people down, causing
real hardship as a result.
"Our report shows there is no excuse for this level of failure."
She added: "We have heard stories of real disruption to people's lives: radio silence on what was happening, businesses shut down and customers forced to make long journeys to pick up bottled water.
"Many customers were effectively left to fend for themselves with local bodies and volunteers having to fill the gap.
"We expect to see concrete improvement plans from companies, particularly those that had the most customers left without supply."
Water UK, which represents firms, said it would be working with them on the report's recommendations.
Its chief executive, Michael Roberts, said that in "some areas significant numbers of customers experienced disruption and hardship and we are determined to prevent this happening again in future".
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