Plans for a tunnel built between Scotland and Northern Ireland have been ditched amid a Treasury clampdown on spending, according to reports. 

Expected to cost a hefty £15 billion, it was deemed "the world's most stupid tunnel" by the PM's former chief advisor, Dominic Cummings. 

Now, the Financial Times has reported that the plans are no longer going ahead with Chancellor Rishi Sunak's autumn budget due next month. 

But what exactly was the proposed tunnel, and how was it supposed to work?

Here's what you need to know...

What was the proposed tunnel between Scotland and Ireland?

Initially, Boris Johnson suggested building a bridge between Stranraer in Scotland and Larne in Northern Ireland back in 2018. 

Stranraer is a town in Dumfries and Galloway while Larne is in County Antrim. 

Johnson's proposal was met with criticism and scepticism from engineers who raised questions over the possibility and safety of building a bridge over a stormy patch of water. 

However, the proposal of a tunnel, like the Euro Tunnel, was then born and considered by transport connectivity review led by the Network Rail chair, Sir Peter Hendy. 

It was the High-Speed Rail Group (HSRG), composed of rail industry leaders, that suggested tunnelling under the Irish Sea between Stranraer and Larne, stating the tunnel would bring Northern Ireland closer to Great Britain and would "address problems in economic status of Northern Ireland post-Brexit". 

The tunnel would be 21-miles long and would require additional rail connections to be feesible.

The report from Hendy is due before the annual budget on October 27, however a tip-off suggests no money will be released to allow plans to go ahead. 

Why have plans for the tunnel reportedly been ditched? 

Rishi Sunak is due to present the next budget on October 27 which according to reports will not include any money designated to the building of a bridge or tunnel between Northern Ireland and Scotland.

When announcing the budget, the Chancellor warned there would be no extra money for departments on top of what was already announced in the March budget. 

It means there will be no extra money designated to the Department of Transport for the Scotland-Northern Ireland tunnel. 

The exception is the health department, which will receive an additional £36 billion from the rise in National Insurance tax to pay for the Covid catch-up and reform of social care.