It had been suggested the new format, which will take place in addition to the 18-team T20 Blast, could start in 2018 if it was given the go-ahead.
But England and Wales Cricket Board (ECB) bosses have told county members' forums this week that 2020 is now their targeted start date.
That is partly because the existing broadcasting deal expires in 2019.
The ECB is keen to introduce a new T20 event to rival the Indian Premier League and Australia's Big Bash League, and has said this is a watershed moment for the sport in England and Wales.
It has said it needs to draw new audiences to the game and believes a new competition would increase revenues for all.
The eight-team proposal, which was one of five on the table, was passed by a majority vote at Lord's in September.
It followed discussions between representatives of the 18 first-class counties, the Professional Cricketers' Association, and the MCC, which is the guardian of the laws and spirit of cricket.
Surrey, Kent and Sussex were reportedly against the move.
A fear games would only be held at Test match venues was said to be behind some of the initial opposition.
But the ECB, which is discussing the plans for the competition at a board meeting on Tuesday, is referring to the new tournament as regional rather than city-based or franchise-based.
That could open the way for some games to be held at non-Test grounds.
Some counties have welcomed the proposal, but the BBC understands others have raised concerns about its potential impact on their revenue and future health.
Several counties have also expressed misgivings about a lack of detail behind the idea, which is still in the planning stage.
Championship toss rule unchanged
The ECB has also decided to extend the 'optional toss' regulation, introduced this summer, for the 2017 season.
It means visiting captains can choose to bowl first without the need for a toss.
If, however, they decide not to exercise that right, a toss will take place in the usual way.
The measure was introduced to prevent home counties preparing seamer-friendly pitches, and to encourage more spin bowling.
This summer, 85% of Championship games lasted until the fourth day, compared to 74% in 2015, and 843 wickets were taken by spinners, an increase of 91 on the previous year.
A proposed eight-team Twenty20 competition in English cricket may not start until 2020.