Ofgem has asked suppliers to review the use of court warrants to enter the homes of customers in arrears.
Prepayment meters are more expensive than direct debit, but are sometimes the only option for people who struggle to pay.
Chris O’Shea, boss of Centrica which owns British Gas, said he was accountable for the contractor’s behaviour, but Mr Brearley head of Ofgem said it was astonishing that any supplier would not know.
Mr Brearley warned British Gas that it could face the strongest action needed if it violated its legal and moral responsibilities.
Graham Stuart, the minister for energy and climate, said British Gas should “hold their heads in shame” for how some customers had been treated.
Caroline Flynn, former shadow energy secretary, said forced installation of prepayment meters had been raised with the government last year following a surge at the end of summer.
Jane, who had lost her unborn child at 17 weeks, came home to find someone had been in her home and installed a prepayment meter and left a letter in the kitchen.
In certain circumstances, energy firms can put at-risk customers on a prepayment meter, but they must have exhausted all other options before doing so.
Dame Clare Moriarty said Ofgem’s announcement exposed the rotten core of debt collection practice in the energy sector.