The recently opened Shard is already a familiar landmark on the London skyline, but how might the UK's urban landscapes look if some of the most architecturally ambitious plans of past centuries had been fully realised?

Economic and social factors across the ages meant that some of the grandest designs of renowned architects such as Lutyens and Inigo Jones were never completed.

Here are five ambitious building projects that never made it off the drawing-board.

Liverpool's Catholic Cathedral

The famed architect Sir Edwin Lutyens was commissioned in 1929 to build a new Catholic cathedral in Liverpool. The building he planned was monumental.

"It would have been 60ft (18m) higher than St Peter's in Rome, it would have been twice the height of St Paul's in London," says Anthony O'Brien, Dean of Liverpool Metropolitan Cathedral, which now stands on the original site.

Lutyens' aim was to build a vast brick and granite cathedral topped with a 510ft (155m) dome. The cathedral was to be perched on a high point in the city, its vantage point and sheer size would have dominated the skyline.

"Right from the beginning, Liverpool Cathedral seemed incredibly ambitious. Lutyens was always intensely competitive in his work," says Jane Ridley, biographer and great-granddaughter of the architect.

House of God Sir Edwin Lutyens, proposed architect of Liverpool's Catholic Cathedral was Anglican

Liverpool's Anglican Cathedral designed in 1903 by Giles Gilbert Scott - a Catholic

Foundation stone of Lutyens' plan laid in 1933 but building never finished

Lutyens also designed large sections of Delhi

Lutyens died New Year's Day 1944

A campaign was launched among the city's Catholic community to raise funds.

"In order to finance the building, the parishioners were asked to donate any old gold or jewellery that would contribute to the cost of the building," says Geraldine Judge, a church community worker in Liverpool.

Work began in 1933 with the building of the crypt. That's as far as it got.

"The crypt is complete. It is a great Lutyens building in its own right. It is staggeringly big. It is like walking into a colossal Edwardian railway terminus," says broadcaster Jonathan Glancey.

Why didn't it happen?

The cost was always prohibitive, but work halted with the outbreak of World War II and as the conflict progressed Lutyens lost enthusiasm.

The grand project ran out of steam after his death in January 1944.

Edinburgh's grand classical avenue

South Bridge in central Edinburgh is a traffic-heavy road packed with discount shops - a far cry from plans drawn up in the 1780s for a grand, classical avenue that would have provided a welcoming entrance to one of the great cities of the Age of Enlightenment .

South Bridge Plans followed construction of Edinburgh's North Bridge in 1772

Bridge required to cross a ravine

Designed by Robert Adam, charged £1200 for his design but only received £900

Adam died in 1792

Architect Robert Kay went on to complete South Bridge

Designed by Robert Adam, a giant of 18th Century architecture, the avenue would have included assembly rooms, tea-rooms, colonnades and town-houses.

The avenue was never fully realised but parts of the project were built.

Adam's Register House shows the architect's vision for an "Enlightenment Edinburgh" says historian Nicholas Phillipson.

"There was a sense that Edinburgh would change. There was a shortage of houses and of public buildings. The city had lost its parliament and aristocratic elite. Where would Edinburgh go?" says Phillipson.

Adam's vision was to create one of the most beautiful streets in Europe - on a par with Renaissance palaces of northern Italy.

Why didn't it happen?

Although he was a renowned architect, Adam had a reputation for over-spending.

"Adam charged £1200 for his designs alone, so I dread to think how much it would have cost to produce the finished article," says Frances Sands, curator at the Soane Museum.

Find out more Listen to architect Jonathan Glancey's journey through Unbuilt Britain on BBC Radio 4 at 13:45 BST each day this week

Catch up with episode one on Liverpool Cathedral on iPlayer via the link (UK only)

Adam was asked to build South Bridge but ended up redesigning parts of the city, says Sands.

"It was a little unrealistic to think anyone would have the money to pay for this scheme, but that is characteristic of Adam, who did rather get carried away," says Sands.

Edinburgh's city fathers knew they could get a cheaper scheme, says Iain Gordon Brown of Edinburgh World Heritage .

"They wanted utility, rather than grandeur. They wanted something functional. They wanted a viaduct with houses and shops which could be let to provide rates.

"Adam's scheme was not built but the pattern of it survives, elements of the plan are evident in Edinburgh today in Hunter Square and Blair Street, preserving the ghost of what might have been," says Brown.

Inigo Jones's Whitehall palace

Imagine a monarch's residence in central London that would dwarf Buckingham Palace.

It was almost a reality and perhaps the most ambitious design never built.

Whitehall Palace - to replace the earlier complex of the same name - was the dream of Inigo Jones, the first English architect to design in the Classical style.

Jones's plan aimed to create a huge palace covering the area now known as Downing Street, Horse Guards Parade, Whitehall and large swathes of St James' Park.

James I was keen to bankroll Jones's grand scheme. The first part - Banqueting House - was built in 1622 and still stands on Whitehall. Other fragments survive under Downing Street, says Simon Thurley of English Heritage.

"This was the first building in England in which the rules of Classical architecture were properly understood and it was far bigger than the then-standing Tudor Whitehall palace.

"James I was incredibly ambitious for London and the monarchy architecturally," says Thurley.

"The Banqueting House hall is big enough to be a bus garage, you could get eight double-decker London buses into it and that was less than 5% of what Jones wanted to build."

Why didn't it happen?

In the early days of his reign, James I's son and successor - Charles I - was keen to continue the project but struggled to raise funds.

Whitehall Palace Palace on the site from 13th Century

First parts of Jones's renewal commissioned by King James I

Inigo Jones submitted further plans in 1638

Jones's design covered much of what is now Whitehall and St James' Park

Much of Tudor-era palace destroyed by fires in 1690s

Henry VIII's covered tennis courts survived and now form part of Cabinet Office building

"The king was always fighting with parliament about funds, which was what eventually caused the English Civil War of 1642, so [this] was probably a doomed project from the very start," says historian Michael Leitman.

"What is astonishing is that Charles I was trying to get this palace built in the lead-up to the Civil War, when everything was collapsing around his ears," says Thurley.

"He was captured and locked up in Carisbrooke Castle on the Isle of Wight and still called Inigo Jones to visit him. Together they designed this palace."

Charles I was later beheaded under the orders of Oliver Cromwell in January 1649.

He died in front of Banqueting House.

London's pyramid of death

Today, Primrose Hill in north London is a park offering views of the city.

But in 1829, with a squeeze on burial space in the capital, architect Thomas Willson planned a pyramid mausoleum to hold up to five million bodies.

Willson, born in 1780 and trained at the Royal Academy, wanted the pyramid to be 94-storeys high and cover a site of 18 acres, (7000 sq m).

"It was supposed to be compact, hygienic and ornamental," says Catharine Arnold, an expert on London's dead.

"Willson hoped people would come to admire this huge pyramid from far and wide, picnicking on Primrose Hill and enjoying this splendid monument. But it would be rather like a giant car-park of the dead," says Arnold.

The pyramid-design caught the public's mood for Egyptiana - the height of fashion at the time. Winding walks, similar to today's Guggenheim Museum in New York, were planned to transport the bodies through the pyramid's catacombs to their final resting places.

Why didn't it happen?

"It would have been monumental, but grotesque - a literal Valhalla," says author Simon Jenkins.

"Public opinion stopped it. The arguments for leaving the site wild won the day and Primrose Hill became one of London's most popular parks."

Pyramid Mausoleum Planned to ease London's lack of burial space

Designed by early Victorian architect Thomas Willson to hold 5m bodies

The pyramid was to be 94-storeys high

Projected completion cost of £2,500 in 1829

Willson claimed the pyramid would make £10m profit when full, but there was suspicion among the authorities that his figures did not add up.

There was also concern that the weight of the bricks may have crushed Primrose Hill. And the public mood turned against necropolises - cities of the dead - as garden-style cemeteries became the norm.

With the building of Highgate cemetery in 1829, the pyramid scheme was finally shelved.

McCaig's tower, Oban, Scotland

McCaig's Tower stands on a hillside overlooking Oban in Argyll, Scotland.

Planned by local businessman John Stuart McCaig as a Scottish version of the Coliseum in Rome, many now refer to it as McCaig's Folly.

Situated on two acres of ground, 230ft (70m) above sea-level, it was designed by McCaig himself. Work began in 1896 - at a cost of about £6000 - but it was never completed.

McCaig's Tower Coliseum-style construction designed and funded by John Stuart McCaig in 1896

Intended as a monument to the McCaig family

Proposed cost estimated at £6000

Views from the tower across to Inner Hebrides

Tower part never built and build finally shelved in 1915

It's a Gothic unfinished coliseum above a small Scottish highland port. It's very strange," says Johnny Roger of the Glasgow School of Art.

"It's a personal vanity project. McCaig was going to fill this coliseum with statues of himself and his family, not heroes of Scotland's past."

But the politically radical McCaig was also a great philanthropist, and he also wanted to provide work for local stonemasons and artisans at a time of great unemployment.

Why didn't it happen?

McCaig died in 1902 and building stopped.

McCaig left about £60,000 - £6m today - in his will, and instructions that the tower be completed by local workmen.

His sister did not want to finish the building and successfully appealed the legacy. The judge, some believe, opposed McCaig politically and ruled against the tower on the grounds that the public would have no right of access.

"The criticism levelled at McCaig was that he had done nothing for the community. That was transparently not so and the only explanation must have been political - that the judges did not approve of his politics," says Michael Moss, historian at the University of Glasgow.

Further court rulings in 1915 halted the project and the coliseum, without its tower, stands incomplete.