The Department of Transport is understood to have proposed a specific plan to the Treasury, in order to release preparatory and development funds worth up to £1.5bn to allow for building work to begin around 2030.
But it is understood more time is being taken to process an "extended tyre kicking" of the plans to ensure that the project has been fully scoped out before pushing ahead - a HS2 mistake the government wants to avoid.
"We are determined to learn from the mistakes of the past and not repeat the previous government's failures over HS2 and the botched Network North plan," the government source added.
"That's why we're taking our time to get this right. We will set out our plans in the coming weeks."
HS2 has been beset by setbacks, leading to missed deadlines and ballooning costs, and will now only run from London to Birmingham, with previous plans to go further north scrapped.
In June, the government said the opening of HS2 would be delayed beyond the target date of 2033, but it did not say when the high speed line will begin operating.