Boris Johnson is considering plans to put the southern half of High Speed 2 on ice amid mounting concerns about the costs of the scheme.

The Tory leadership contender, who is expected to enter Downing Street on Wednesday, has been studying proposals to begin building the railway line in the North, rather than London, as part of a set of policy changes designed to demonstrate a focus on "left behind" regions and towns.

The plans, discussed as part of his team's "transition" work in readiness for taking office, would involve pushing back construction of the southern half of the line, between London and Birmingham, and giving the green light to the phase connecting the West Midlands with Leeds and Manchester.

This week he is expected to make a major speech in the North, which is likely to feature a commitment to boosting local transport connections, including east-west rail links under the Northern Powerhouse project.

The disclosure comes after it emerged that Allan Cook, the chairman of HS2 Ltd, the government-owned firm behind the scheme, has warned Chris Grayling, the Transport Secretary, that the project cannot be completed within the official £56 billion budget.