Boris Johnson is poised to give the formal go-ahead to the HS2 project on Tuesday, with ministers promising an additional £5bn on buses and cycle routes to head off critics who fear the troubled high-speed rail line will suck cash from other priorities.

Before the expected announcement giving the green light to the vast infrastructure project, Johnson will claim that his government will offer bus passengers outside London a more frequent service and simpler fares, as part of his agenda to “level up” the UK.

Arguing that local transport could have a “truly transformative role to play in levelling up infrastructure across the country”, Downing Street claimed the investment package in bus transport of £5bn over five years would result in more frequent services, including on Sundays, a simpler fare structure, and new priority schemes to allow buses to skirt traffic jams.

It said many more bus routes would become “turn up and go” services, like those in London.

The PM’s long-awaited announcement declaring his intention to press ahead with the controversial HS2 plans – now with an estimated price tag of more than £100bn – is expected to be balanced by a plan to change the governance of the project in a bid to ensure value for money.

It was unclear whether the government is planning to legislate to allow local authorities greater control over bus services – with further details promised in a “national bus strategy” later this year.

Margaret Thatcher’s government deregulated bus services outside London in the mid-1980s, arguing that competition would lead to increased passenger numbers, by reducing fares and improving services.

Instead, a report by the cross-party transport select committee found last year that “bus use has declined year after year, and successive governments have made no concerted or coordinated effort to reverse or even to stem the decline. In fact, the 30-year policy of deregulation, outside of London, has made the situation worse in most areas.”

Local authorities are able to subsidise non profit-making routes, but many have made deep cuts in the past decade as their budgets have been squeezed.

Grant Shapps, the transport secretary, said: “We want everyone across this country to have the transport services they need to improve their lives and opportunities – to do that, we must invest in the here and now.”

With new Conservative MPs in former Labour seats hoping to show their constituents a tangible benefit to their vote, the announcement will be sold as an immediate measure to improve transport outside of London long before plans for HS2 – the first phase of which is scheduled to open in 2028 – come to fruition.

Proposals for the Y-shaped HS2 scheme run from London to Birmingham, and on to Manchester and Leeds. They are intended to free up capacity on local lines, and form the spine of a rail network fit for the 21st century. But its expected cost has rocketed since it was first announced by the Tory-Lib Dem coalition government in 2012-13.

Johnson’s most senior adviser, Dominic Cummings, was known to be sceptical about the merits of the costly scheme, as are a number of the new-intake Conservative MPs, who have lobbied instead for the money to be spent on local transport schemes.

But northern council leaders and city mayors, including Birmingham’s Andy Street, have lobbied hard for the plan: though Andy Burnham, the Labour mayor of Manchester, has argued that northern powerhouse rail, which would link northern cities, should be prioritised.

The decision to press ahead had been widely expected since the chancellor Sajid Javid threw his weight behind HS2 last month, telling allies it was essential to the government’s plan to “level up” regions outside London.

And Johnson appeared to let slip his intentions to a 10-year-old schoolboy who interviewed him last month, telling him, “in a hole the size of HS2, the only thing to do is keep digging.”

Anticipating the decision, Labour’s shadow transport secretary Andy McDonald said: “HS2 has been appallingly mismanaged by the Conservative party, which has failed to deliver a single major infrastructure project on time or within budget.

“If HS2 is going to get back on track, the project has to be integrated with Crossrail for the North, it needs to be managed as part of an advanced rail network, and it must eventually extend into Scotland so that we remove the need for domestic flights.”

Cabinet will meet on Tuesday morning to give its final approval for the plan, though Downing Street stressed that there was no necessity for formal sign off.

As well as the boost to bus services, Johnson will also promise a relatively modest 250 miles of new segregated cycle routes, and “dozens” of “mini-Holland” schemes aimed at increasing cycling and walking, like those he championed in several London boroughs.

The commitment of 250 miles nationally over five years compares to a blueprint for 1,800 miles currently being planned by Greater Manchester under its mayor, Andy Burnham.

The “mini Holland” schemes involve changes such as closing residential roads to through-motor traffic but allowing free access for cycling, to boost bike use and walking.

Both the cycling and bus schemes are the work of Andrew Gilligan, the former journalist now at No 10, who was Johnson’s cycling tsar in London. There had been expectations that he was planning more ambitious cycling provisions.

The £1bn a year funding for the bus and cycling plan is expected to be funded predominantly from extra borrowing.

Javid’s budget next month will signal a significant loosening of the purse strings, as the treasury adopts new spending rules that will allow the chancellor to use increased government borrowing to pay for long-term projects.