The cost of bringing Crossrail to Kent has more than doubled – and could yet end up costing more – amid growing pressure to deliver the project for hard-pressed county commuters.

Transport for the South East (TfSE) has set out its strategic plans for the region's transport network which involves pumping £45bn into everything from buses to road and rail links.

Among the transport body's most ambitious goals is to bring the Elizabeth Line – the new underground route – to Kent via Ebbsfleet.

Last month saw the long-awaited opening of the £18.9bn Crossrail project, operating between Abbey Wood and Paddington.

But while the "purple one" is expected to relieve some pressure on the north Kent line, Dartford still has some of the slowest commutes recorded inside the M25 commuter belt. For example, it takes a sluggish 52 minutes to access central London when not using the Tube for part of the journey.

Patchy rail coverage in and around the town could soon be on par with Dover – more than 60 miles away on the coast – if ambitions are realised to deliver a proposed "under-the-hour" High Speed rail service from the port town to the capital.

Crossrail has long been mooted as a means of improving rail connectivity in the north Kent corridor. It's believed the move could also drive a modal shift away from cars and address bottlenecks at the Dartford Bridge.

The original passage of the purple fleet would have seen the Elizabeth line terminate at Ebbsfleet.

But the idea was shelved in 2004 owing to cost pressures and infrastructure difficulties involved in the Tube sharing tracks with the Southeastern and National Rail network.

Over a decade later the Crossrail to Ebbsfleet (C2E) Partnership was formed as an informal group of local authorities and transport agencies to reconsider a 16km extension towards Ebbsfleet, as well as supporting the delivery of new homes and jobs.

In 2019, the partnership was handed nearly £5m in government funding to produce a study exploring the five best options – later whittled down to three.

Of those considered, two involve enhancing the Elizabeth Line to provide more direct rail services from London to Ebbsfleet, Northfleet and Gravesend.

In each case, some sections of additional track would need to be built in addition to junction works, enhancement of existing stations and building new stabling facilities.

In October, the C2E Partnership submitted its strategic outline business case to ministers and set out its preferred choice. This would see eight of the 12 Elizabeth Line trains per hour that currently terminate at Abbey Wood extended eastwards to Ebbsfleet.

However, where the proposal was initially costed at £1.5bn, the current cost now sits between £2.6bn and £3.2bn, according to a new report by TfSE.

That price is for the preferred choice and TfSE notes "there are a range of different options under consideration in the business case, some of which may involve a higher cost".

If approved the extended route could have a Crossrail/HS1 interchange and give Kent commuters direct access to Heathrow Airport.

Kent University professor Richard Scase, a business expert in economic trends, believes there is a "strong strategic case" for extending the Tube towards Kent.

"From an economic point of view it makes complete sense," the emeritus professor said.

"It really could have a massive impact and be a real hub for economic growth."

While many people move from Kent to the capital, a significant number of people are moving in the opposite direction in search of more affordable housing and a better quality of life.

This "ripple effect" has been attributed to tight planning constraints in building new homes in outer London.

Targeted transport improvements – such as a Crossrail extension into Ebbsfleet – could further encourage Londoners to move to Kent and benefit from the high-quality travel links it offers, Prof Scase adds, leading to thousands of "high skilled, dynamic" jobs in line with the government's levelling up agenda.

The business forecaster believes opportunities are being missed at the Garden City – where up to 15,000 homes are planned – to tap into the east London tech cluster.

As with all major and complex infrastructures projects finances will be the main sticking point.

TfSE's report suggests a wide range of funding sources could play a role beyond central government funding.

It would require Great British Railways, a state-owned public body to adopt the project in recognition that much of the works are on the existing north Kent line.

The Department for Transport (DfT) would then need to authorise and progress the project before any government funding can be committed.

As part of a bespoke package, external capital could be sourced from broader programmes that "recognise the scheme’s potential to contribute to national housing, economic and environmental objectives".

Transport for London and the Mayor of London could also be asked to cough up cash but this might prove problematic given the transport body's ongoing financial struggles.

Lastly, local authorities may have a role to play but are likely to be impacted by significant challenges of "political and community acceptability".

"There is a good strategic case for it but you suspect the business case will be weaker," Prof Scase hastens to add.

He suspects the project could end up costing more than the figures quoted given escalating building and labour costs and the current economic landscape.

But the expert believes it would be short-sighted of the government to overlook the long term benefits of extending the Elizabeth line.

He points to missed opportunities such as the then London Mayor Boris Johnson's suggestion to build an airport in the Thames Estuary as an alternative to a third runway at Heathrow.

An airport off the Kent coast has been mooted for years with other possible sites off the coast of Sheppey, and the Isle of Grain discussed. But the proposal, dubbed "Boris Island" was ditched amid strong local opposition.

The Department for Levelling Up, Housing and Communities and the DfT are currently considering the business case for the Elizabeth line extension.