Enabling work recently began on the longest tunnel ever built under the Thames. It won't be in London, but it will have a big effect on the eastern side of the capital. Here, then, is your simple Explainer for the Lower Thames Crossing.
Where is the Lower Thames Crossing?
Nowhere yet. It's still to be built. But in a decade or so the 2.6-mile twin-bore tunnel will connect Essex and Kent, diving under the Thames just east of Tilbury and Gravesend. This will be the longest road tunnel in the UK.
It's not just a tunnel, though. Around 14.5 miles of new road will connect it to major routes either side of the Thames. To the south, the link will reach the A2 and thereby M2; to the north, it'll meet the A13, then carry on to reach the M25 between Junctions 29 and 30.
Who's building this and why?
This is the work of National Highways, the government-owned company responsible for building, improving and maintaining the country's motorways and major roads.
The tunnel is intended to relieve the Dartford Crossing around seven miles to the west. This is the only fixed crossing outside of London to the east. A second crossing would relieve congestion on what National Highways describe as "one of the country's most unreliable roads", effectively doubling capacity.
It's not technically a motorway (it will carry the name A122), but will function much as a smart motorway, with three lanes in each direction and variable speed limits. National Highways describe it as "the most ambitious road project since the M25".
That's one way to look at it, "horrendous scar on the landscape" is another
These things are complex. Digging up miles of countryside to lay down tarmac, then populating it with cars and trucks is a massive, massive assault on the local environment, no two ways about it. Habitats will be lost, trees will be cut down, before we even get to the carbon cost of the construction works and extra emissions from 'induced demand'. The Woodland Trust, Essex Wildlife Trust and Greenpeace have all opposed the plans, along with many other groups.
National Highways would point out that the project will plant more trees than it destroys, including 260 hectares of woodland (though these will take many years to mature). Formerly monoculture farmland will be rewilded for biodiversity. And the whole point of the project is to relieve the congested Dartford Crossing, thereby helping alleviate pollution build-up in that area.
We should also remember that the roads won't open until well into the 2030s, by which time we might expect a much higher percentage of electric vehicles on the road, and therefore lower emissions.
How is this funded?
The project is currently budgeted at £10.6 billion. That's about a tenth of the estimated bill for HS2, though more expensive on a per-mile basis. We're all paying towards it. As things stand, the Government (i.e. taxpayers) are contributing £3.1 billion, while £7.5 billion is coming from private funding.
Why would private investors pay the lion's share of a massive road/tunnel project?
Without getting into the complexities of finance, the key to it all is tolls. The new tunnel will be tolled, presumably at the same level as the Dartford Crossing charge. Investors will get a share of the pot from both crossings over a long period. The Dartford Crossing paid for itself a long time ago and is now generating handsome profits (to the consternation of drivers), which are ring-fenced by the government for transport upgrades. This money will instead go to the private investors.
The identity of said investors — likely to be pension funds, sovereign wealth funds and the like — has not been announced.
How are things progressing?
The government approved the project in March 2025, and fully committed to the public side of funding towards the end of the year, allowing construction to begin.
The earliest enabling works began in March 2026. Archaeological teams are out inspecting the land either side of the river, utilities are being diverted, and storage compounds for machinery are being built. Teams are also getting to work on creating new wildlife habitats, to mitigate against some of the project's later upheavals. Major construction, including tunnelling, is due to begin in 2028.
How long will this take, and when will it open?
In the unlikely event that the project encounters no major snags, then construction should be complete in "the early to mid-2030s". Last time we wrote about this project back in 2018, it was due to "open in 2027", so adjust your expectations accordingly.
We're going to stick our neck out and randomly predict 26 July 2036.
Can you tell me something mildly amusing about the Lower Thames Crossing?
Yes, the southern connecting road will join the A2 beside the tiny Kentish village of Thong. The junction looks like a thong. This is going to keep us amused for decades.
Find out more on the Lower Thames Crossing website, where they most certainly do not snigger about thongs.
