Boris Johnson has indicated that his long-held hopes of building a tunnel connecting Northern Ireland to Scotland have been abandoned for now after Treasury pushback.

       Mr Johnson told reporters during his US trip that proposals for the connection remained "an ambition" but were "not the most immediate".

       He also said it would only happen after the completion of the HS2 rail link, which under current plans would not be fully open for another decade. Fresh doubt was also cast on whether HS2 will be scaled back amid reports that the Treasury has become frustrated about its ballooning budget and is willing to act to limit it.

       Mr Johnson has long envisaged some form of road or rail transport connection across the Irish Sea as a way to tie the UK closer together. He asked Sir Peter Hendy, the Network Rail chairman, to include the idea of an Irish Sea fixed link in his review of the UK's road, rail and air infrastructure.

       But the project, which could cost around ￡15 billion, was reportedly due to be killed off by the Treasury as it conduct this year's spending review, with decisions announced next month.

       ‘It will be wonderful for all parts of the North’

       Mr Johnson was asked about the tunnel and whether the second leg of HS2 from Leeds to Manchester could be scrapped – another cut the Treasury is reported to be considering.

       He said: "You'll have to wait until the spending review and the integrated rail plan, which is also coming out. But it will be wonderful for all parts of the North, the North East, North West, Leeds, Nottingham, Derby, everywhere, Newcastle, Liverpool, Manchester – we have wonderful plans.

       "What I would say perhaps about the tunnel/bridge is perhaps although it remains an ambition, it's not the most immediate. It will be delivered substantially after the rest of the programme that you have just described."

       The first phase of HS2 between London and Birmingham is due to open at the end of 2026. The second, between Leeds and Manchester, was scheduled to be completed by 2033/34.

       It means, realistically, that Mr Johnson, who took office in 2019, is unlikely to be in power to deliver the tunnel – a hint that he is giving up hope on the project for now.

       Dominic Cummings, Mr Johnson's former senior Number 10 adviser, called the plan "the world's most stupid tunnel to Ireland".