Tunnelling work under London for the £14.8-billion (20 billion euros, $23 billion) Crossrail project was completed on Thursday in a major milestone for Europe's biggest construction project.

Prime Minister David Cameron and London Mayor Boris Johnson attended an underground ceremony to mark the final breakthrough under London's financial quarter after 26 miles (42 kilometres) of tunnelling.

Standing next to "Victoria" -- one of the eight giant boring machines used for the digging -- Cameron said that the project was "absolutely magnificent" and hailed it as an "engineering triumph".

"Crossrail is an incredible feat of engineering that will help to improve the lives of working people in London and beyond.

"The project is a vital part of our long-term plan to build a more resilient economy by helping businesses to grow, compete and create jobs," he said.

Johnson, who like the prime minister wore orange protective clothing, said the tunnel completion was "a huge success for the whole of the UK economy".

The tunnelling, which began in 2012, involved the machines boring their way through the capital, dodging existing tunnels, sewers and underground rivers, at a rate of up to 100 metres a week.

London's mayor praised the "Victoria", which weighs 1,000 tonnes and measures seven metres in diameter, as a "monumental muck-munching machine" at the underground ceremony attended by some 150 workers.

More than 10,000 people work on Crossrail.

Millions of tonnes of the excavated soil have been shipped off to create a nature reserve for birds on an island in the Thames Estuary east of London.

Crossrail will link towns west and east of the capital, providing a vital connection for commuters who rely on London's overcrowded transport system.

The first trains, which will travel overground outside London, are planned to run from 2018.

- 'Temporary respite' -

At Thursday's event, Crossrail chief executive Andrew Wolstenholme said the project would "transform how people travel across the capital".

But some observers warn the project will not do enough to ease congestion due to an ever rising number of visitors and a fast-growing population.

London's Evening Standard in an editorial this week called for more spending on the London Underground and a new Crossrail 2 line, warning Crossrail "will offer only a temporary respite from overcrowding."

"We can be proud of London's pre-eminence as a world city, and of the wealth it brings the capital and the wider economy. But we cannot take it for granted: to keep the visitors coming, we have to invest."

At the underground completion ceremony, Johnson also called for support for Crossrail 2, a north-south line that would cost as much as £27.5 billion.

As part of the current project, around 3,000 skeletons dating back to the 16th century and including plague victims, have been removed from a former burial ground close to Liverpool Street station.

Archaeologists said they hoped to piece together a microcosm of old London and to test DNA from people who died of outbreaks of plague for clues about how to fight the disease.

Deeper in the excavation, the archaeologists found the remains of a Roman road, as well as horseshoes, hairpins and burial urns.