There had been doubts over the project after opposition and spiralling costs.
Boris Johnson has given the go-ahead to the £100bn flagship rail project High Speed 2.
He announced work on the hugely expensive scheme will continue after carrying out a review into whether it should proceed given costs have "exploded".
However Phase 2B, which runs from Birmingham to Leeds and Manchester, will be reviewed and subsumed into the current "Northern Powerhouse Rail” project.
The Prime Minister said the firm handling construction of HS2 has not "distinguished itself" but that "poor management has not detracted from the fundamental value" of the plan.
There is "no doubt of the clinching case for high speed rail", he added, promising to "restore discipline to the programme" by appointing a full-time minister to oversee its progress in Thursday's cabinet reshuffle.
Whistleblowers are calling for a public inquiry and furthermore, the plan puts the PM on a collision course with up to 60 Tory MPs who oppose the project due to disruption in their constituencies.
He confirmed work on phase one - which runs from London Euston to Birmingham - and Phase 2A - an extension to Crewe - would continue "on something approaching time and budget".