Supporters of HS2 are growing increasingly anxious that Boris Johnson is prepared to scrap part of the high-speed railway project or overhaul it in a bid to redirect the money towards other transport in the north of England.

Local government leaders from the north and Midlands are privately concerned there has been a change of tone from No 10 since Christmas, causing nervousness that the government could scale back the scheme.

One pro-HS2 source with knowledge of Downing Street’s thinking said the future of the project, first announced in 2009, was “hanging in the balance” since the election campaign. Johnson’s transport adviser, Andrew Gilligan, is known to be against HS2, and Dominic Cummings, his chief adviser, is also not keen, having described it last year as a “white elephant”.

The parts most under threat are thought to be the first leg between London and Birmingham or even the eastern leg from Birmingham to Leeds, with billions potentially redirected towards other transport links in the north and Midlands such as the proposed Northern Powerhouse rail project. This is a plan for a fast rail route between Manchester and Leeds, as well as potentially other northern cities.

Johnson has for months been expected to endorse HS2 if it can reduce its costs, after commissioning a review by Douglas Oakervee, which is understood to support the whole line going ahead.

However, the government is dragging its feet over the publication of the Oakervee report and final decision, claiming the document is not finished yet even though it was submitted to the Department for Transport in November.

A DfT source insisted “there is no final report” as Grant Shapps, the transport secretary, “had some questions” about earlier drafts and it was sent back for revisions. No 10 said Johnson was not in a position to make a decision because he had not yet seen the report.

At a packed meeting in parliament on Tuesday, council leaders from across the north of England and Midlands said it was crucial that the project went ahead.

Judith Blake, the leader of Leeds council and chair of the Core Cities project, said: “What we’ve seen in Leeds is just the promise of HS2 coming in has been transformational in itself … I want to make it absolutely clear that we can’t talk about Northern Powerhouse rail or HS2. We need the two together.”

Sir Richard Leese, the Labour leader of Manchester city council, said it was clear that he, Blake and the Tory West Midlands mayor, Andy Street, were all “singing off the same hymn sheet” in their calls for HS2 to get the go-ahead.

“When we get down to what investment we need, we should not be making choices between local services, between Northern Powerhouse rail and HS2. The north of England is absolutely fed up of getting the crumbs from the table,” Leese said.

Street said: “I’m probably the only person in the room that knows exactly what the Oakervee [report] says because I sat on the review ... What I am utterly confident of is that there is a strong economic case that will eventually win through. The reason there is a strong economic case is that we can already see the benefits on the ground in the West Midlands.”

Chris Heaton-Harris, the rail minister, told the meeting that he could see both HS2 and Northern Powerhouse rail were important but added: “The transport secretary has spoken to Doug Oakervee and is now waiting for the final report … Only once we have the data from an independent source can we make a decision about its future. We hope to make that decision shortly.”

Afterwards, Lord Kerslake, the former head of the civil service, who chaired the meeting, said: “The turnout at the meeting was high, which does suggest people are concerned. There is a concern that the repricing that’s going on with HS2, which was always going to come, will be used as a reason to stop the scheme or curtail it, such as the eastern leg not getting built.

“A second issue is that this is being posed as ‘you can have HS2 or you can have Northern Powerhouse rail but not both of them’. And there is very strong feeling that it should not be either or – you’ve got to do both. And the third point is that, pending the Oakervee report, we are not getting government being full-heartedly behind the project.”

Lobbying efforts on both sides of the debate have significantly stepped up in recent weeks. Council leaders, mayors, a group of Birmingham MPs and the former chancellor George Osborne have all publicly urged the prime minister to go ahead.

Meanwhile, a group of newly elected Tory MPs called on him to scrap the project. Lord Berkeley, who was deputy on the Oakervee review, released his own minority report saying he did not endorse the conclusions of the rest of the panel, warning that costs had spiralled to more than £100bn.

Dame Cheryl Gillan, a Conservative former cabinet minister and leading critic of HS2, said: “I think it is time to reprioritise … We’ve got routes from London to Birmingham; that shouldn’t be a priority. It’s Birmingham northwards that should be the priority. I would absolutely freeze the London to Birmingham stretch and put it on ice.”

A study by the National Trust and the Woodland Trust and wildlife bodies found this week that HS2 will destroy or irreparably damage five internationally protected wildlife sites, 693 local wildlife sites, 108 ancient woodlands and 33 legally protected sites of special scientific interest. This was disputed by HS2 Ltd, which said the data was not new and that it included all sites within 500 metres of the line regardless of how they were affected.

On the future of the project, an HS2 Ltd spokesperson said: “We await the publication of the government’s official review into HS2. Investment in a state-of-the-art high-speed line is critical for the UK’s low-carbon transport future, will provide much-needed rail capacity up and down the country, and is integral to rail projects in the north and Midlands which will help rebalance the UK economy.”