The sale follows the decision last year to sell off a 60 per cent equity stake in its UK gas transmission and metering business to a consortium which included Macquarie and British Columbia Investment Management Corporation from Canada.
The investment takes the Consortium's total ownership in National Gas to 80 per cent and is on equivalent financial terms to the original 60 per cent acquisition, which was completed in January this year.
Final completion of the new deal will not occur until regulatory approval is confirmed, although Macquarie said it anticipates that it will be completed before the end of the year.
The Consortium has also entered into a new option agreement with National Grid for the potential acquisition of the remaining 20 per cent shareholding in National Gas. This option will expire by the end of July 2024 if not adopted.
Following the announcement of the initial acquisition last year, National Grid chief executive John Pettigrew said that the new owners have lots of experience owning big infrastructure and a "long-term commitment to the UK".
The future of the UK's gas network is currently in flux due to differing ideas on how to decarbonise the country's energy supplies. One school of thought advocates abandoning gas for cooking and heating homes in favour of heat pumps and electric cookers that can be powered using low carbon energy.
But others, including stakeholders of the gas networks, want to upgrade the pipes so they can pump hydrogen into British homes instead.
Martin Bradley, european head of infrastructure for Macquarie Asset Management, said: "This additional investment underlines our commitment to National Gas and the critical role it plays in the UK's energy system.
"We have been working closely with the National Gas team since January as they have continued to meet the energy needs of millions of households and businesses. We are pleased to continue our strong relationship with National Grid, and aspire to acquire the Remaining Interest in due course."
National Gas became an independent company earlier this year following the completion of the Consortium's initial investment in January 2023. The firm operates the UK's 7,600 kilometre national transmission system, transporting the gas needed to heat homes and power industry and electricity generation. It also provides maintenance and management services for over 7 million domestic, industrial and commercial gas meters across the UK.
