The Grenfell Tower fire has prompted an investigation into incomplete records of gas pipes in high-rise buildings.
Cadent Gas, which operates and maintains local pipes in four regions of England, said some blocks were omitted from its 10-year survey.
The company alerted energy regulator Ofgem, which is now investigating.
The incomplete records emerged after Cadent received an information request from a council in the wake of the Grenfell Tower tragedy.
Information given by Cadent to Ofgem suggested that 1,000 premises were missed off the records, but Grenfell Tower was not one of them.
Cadent Gas, previously known as National Grid Gas Distribution, is one of four companies that oversee the final leg of piping gas into people's homes, known as distribution.
It has a responsibility to keep records of its network, including risers - the gas pipe arrangement of a building that is more than two floors above ground level and has more than two meter points.
The company owns four of England's eight regional distribution networks - north London, the West Midlands, the North West of England, and the East of England. It said it received a request from one council which highlighted an issue with its records.
"Between January and February 2018, Cadent identified that just some of our records regarding high-rise buildings - building of six storeys and above - were not contained on our digitised records and were therefore not part of our regular 10-yearly survey programme," a spokeswoman for the company said.
"As a consequence, we informed Ofgem at the earliest opportunity in early February and have continued regular progress briefings to date.
"Alongside this, we immediately implemented a survey recovery programme to resolve the issue as quickly and effectively as possible. This programme of work will be substantially completed in July 2018."
Ofgem said it was now investigating whether Cadent kept and maintained records for all of its risers, whether it has the appropriate systems in place to do so and whether it maintains an efficient and economical pipeline system.