A colossal portrait long thought to be of France’s Grand Dauphin that hung for a century in a corridor of the central bank in Paris has been revealed to be British.

The true identity of the sitter, with elaborate regalia and pearl-mounted sword, is not the son of Louis XIV but a lord mayor of London from the late 17th century.

The three-metre tall painting has gone on public display for the first time at Tate Britain, part of an exhibition telling the story of the British baroque period, described by curators as one of the most neglected and overlooked eras of UK art history.

Tabitha Barber, the curator of British art 1550-1750 at Tate Britain, said the painting was purchased by the Banque de France in the 1920s because of its supposed French royal links; it was only recently that a curator at the bank began researching the painting, which was tucked away in a quiet corridor, and discovered the truth.

Barber said there were a number of clues that clearly pointed to it being a London lord mayor: the pearl sword still used today; a background that includes Temple Bar and the dome of St Paul’s Cathedral in London; and the coats of arms on the horse’s ornate trappings.

The painting, by John Closterman, joins a number of new discoveries in the exhibition.

“Of all the periods of art history this is the one people find the most difficult,” said Barber. “Mainly because there aren’t many British names to latch on to. So I think there is a great deal to be discovered about this period, still. I’ve been working on this period for goodness knows how many decades and I’m still being surprised.”

Opposite the Closterman and also going on show for the first time is The Whig Junto (1710) by John James Baker, which had lain unnoticed under layers of dirt in a country house. Cleaned up, it reveals itself as the only known group portrait of a small cabal of politicians who were at the time formulating Whig parliamentary policy. The men are shown meeting at a country house, probably the Earl of Orford’s Chippenham Park estate in Cambridgeshire.

People view The Whig Junto at Tate Britain. Photograph: Neil Hall/EPA

Barber said: “They look as if they are connoisseurs, sitting round consulting antique medals and prints but I think it hides a message about policies to do with war … to continue war with France, rather than the Tory policy of wanting peace.”

Elsewhere in the exhibition are paintings by Michael Dahl of two grand ladies of Queen Anne’s court, part of a series known as the “Petworth beauties”. They are being shown for the first time in 200 years with their legs attached, the result of one of the one of the most unusual and ambitious art restoration projects undertaken by the National Trust.

In the 1820s, when the third earl of Egremont, owner of the baroque Petworth House in West Sussex, wanted more wall space for new works celebrating the Battle of Waterloo he ordered the cutting off of the legs on six portraits in the “beauty room”. Thankfully, the legs were folded round the back of the works rather than severed completely, allowing them to be restored.

“To be honest, I didn’t think we would ever get to this stage,” said Barber. “They look even better than I ever imagined they would. They look stunning.”

• British Baroque: Power and Illusion is at Tate Britain from 4 February-19 April.