The County Championship will feature a 10-team Division One from 2020, with only one team being relegated next year and three going up from Division Two.
The current structure - in place since 2017 - will be reversed, with eight counties to compete in the second tier.
The ECB also confirmed the One-Day Cup will run alongside the new competition - unofficially known as 'The Hundred' - when it launches in July 2020.
Overseas players will not be allowed to play in the 50-over tournament.
The men's domestic playing programme group (MDDP), chaired by Leicestershire chief executive Wasim Khan, met four times before agreeing to the proposals.
"We are very pleased that that there was unanimous support for a structure that will hopefully improve our domestic game and in turn the England teams," Khan said.
The T20 Blast will retain its present format, with four counties qualifying for Finals Day at Edgbaston.
From 2020, promotion and relegation in the County Championship will revert to two-up, two-down.
The ECB say the restructuring will give "an appropriate increase in security" to Division One teams and "increased opportunity" to Division Two sides, with a smaller percentage being relegated and a higher proportion of sides gaining promotion.
"After receiving unanimous agreement we will move forward with plans that will help ensure that our domestic game remains as vibrant as possible while producing players to help our England teams remain successful," said chief operating officer Gordon Hollins.