Britain’s energy suppliers are continuing to use debt collectors to chase unpaid bills after promising to help households during the coronavirus pandemic by offering payment plans to struggling customers.

Under an agreement with the government, energy suppliers are expected to identify customers who might be in financial distress and offer to reassess, reduce or pause bill payments to help “reassure” homes during the coronavirus lockdown.

But many households and small businesses with outstanding balances on their energy accounts may still be targeted by debt collectors with warnings that action may be taken against them if they don’t pay their bills.

A letter sent by a debt collector on behalf of Shell Energy, seen by the Guardian, warns one customer that arrangements may be made for a doorstep collector to visit their home in order to chase an outstanding balance of £78.51.

It warned that Shell Energy would share the account holder’s details with credit reference agencies which could hurt their ability to switch energy supplier, apply for a mortgage or borrow money.

The letter was sent weeks after energy suppliers agreed to support customers who are financially impacted either directly or indirectly by the coronavirus lockdown.

Alok Sharma, the business secretary, brought in the measures to help households which “need additional support and reassurance” during the Covid-19 pandemic. They include an agreement that no customers should be cut off during the lockdown.

Natalie Hitchins, from the consumer group, Which?, said: “Many customers will have been financially impacted by the coronavirus, so it is concerning to hear that any energy supplier is using debt collectors to place pressure on customers who could be struggling during this difficult time, despite agreeing with the government to support customers in financial distress.”

The UK’s largest energy suppliers, British Gas and Ovo Energy, said they were continuing to use debt collectors to engage with their customers. Both companies said they had instructed their debt collectors to offer financial help on their behalf.

E.on UK, which merged with npower last year, said it was not handing any new customer details to third-party debt collection agencies but that its debt collectors would continue to pursue long-standing debts built up before the outbreak.

A spokesman for Shell Energy said a letter from a debt collection company “is a last-resort attempt to engage with a customer and only after we’ve repeatedly asked them to get in contact to see how we can help”.

According to a government spokesman, some suppliers had offered to defer payment deadlines by up to three months and had also suspended following up on debts as part of a package of measures to help households weather the Covid-19 crisis.

“This is a worrying time for people across the country and we would expect energy companies to take the current circumstances into account as they carry out their business,” the spokesman said.

There are also growing concerns over debt collectors who are targeting the UK’s microbusinesses, which are often family-run and may have been hard-hit by the lockdown measures put in place to stop the spread of the virus.

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Ed Dodman, a director at the Energy Ombudsman, said: “What we have noticed in the early days of the lockdown is that the proportion of financially vulnerable microbusinesses coming to us for assistance is higher than it was before the outbreak, indicating that small businesses, in particular, may be struggling with these issues.

“The guidance is quite clear that energy suppliers must consider whether their customers are in financial difficulty and whether they can offer assistance. This is not a carte blanche for customers to avoid paying bills but suppliers should be sensitive to the circumstances of their customers on a case by case basis.”