Miliband asked the National Energy System Operator (NESO) to review ministers’ clean energy plans after Labour’s return to power at July’s general election.
The body was originally owned by National Grid, a private company, but transferred to government ownership last month.
The government is expected to publish its action plan for modernising the grid in the coming months.
Sources within government see its report, to be published on Tuesday, as a vindication of its pledge to deliver a net zero carbon electricity system in Great Britain by 2030, a key part of Labour’s election manifesto.
But it also highlights the scale of the task when it comes to increasing the supply of renewable energy, and connecting it to the grid.
Meeting the pledge, it finds, would require a significant boost to offshore wind capacity, alongside increased electrification of heat, transport and industry.
Nearly 1,000 km (620 miles) of new power lines would be required to connect new renewable energy to the grid, along with 4,800 km of undersea cables, it said.
The report points out that this would be more than double over five years what has been built in total in the last 10, requiring many projects to begin construction in the next 6 months to two years.
It added that this “challenging” timescale will require changes to the planning system to cut down the time new projects take to get approval.
The construction of hundreds of miles of new pylons is also likely to run into political opposition, with a number of schemes facing fierce local opposition.