Up to a billion euros’ worth of treasure has been stolen in a raid on a German museum in the early hours of Monday.

According to reports, the Green Vault in Dresden Museum was targeted by thieves who broke into the building in the early hours of this morning.

The thieves switched off a power supply at 5am before breaking through a window into a museum which once boasted it was ‘as secure as Fort Knox’, it is believed.

According to Bild, they stole jewellery and diamonds which may have been worth up to a billion euros (£850million), although police are yet to reveal exactly what was taken.

The burglars escaped in a saloon car and remain at large after raiding one of Europe’s largest collections of treasures.

However, it is hoped that surveillance cameras may have captured them on video despite the electrical failure.

A fire broke out on a nearby bridge overnight and it is believed that the blaze may be connected to the attack on the power supply.

Shutting off the electricity may have helped the burglars to disable the museum’s alarm systems, it is feared.

Reports in Germany say the thieves were ‘noticeably small’ and able to fit through a tiny space in a window.

Authorities are yet to reveal exactly which items have been stolen or confirm the value of the stolen goods.

Saxony police acknowledged in a statement this morning that ‘unknown’ thieves had broken inside the museum but said further details were not yet available.

State police officers are now at the crime scene as they investigate how the thieves got inside.

‘We have not identified a perpetrator and nor have we yet made any arrests,’ police spokesman Marko Laske said.

A notice on the museum’s website this morning states only that the building is closed today for ‘organizational reasons’.

The museum’s Twitter account confirmed the break-in and said a press conference was due at 1pm German time (12pm GMT).

‘Not only have our state collections but we the people of Saxony been robbed,’ said regional premier Michael Kretschmer.

‘You cannot understand the history of our state without the Green Vault. The treasures found there were made by the hard work of people in our state.’

The security measures at the museum had appeared to be in order and ‘comprehensive’, he said.

A €1billion art theft would be comfortably the largest in history, surpassing the $500million raid on the Gardner Museum in Boston nearly 30 years ago.

Two thieves disguised as police officers stole 13 works of art from the Boston museum in March 1990 and the crime remains unsolved.

The Dresden museum was founded by Augustus the Strong, an 18th-century elector of Saxony, and houses thousands of items including historic coins and jewellery.

However, one of its most valuable treasures – a 41-carat naturally green diamond called the Dresden Green – is currently out on loan in New York.

The museum also houses include a 25-inch figure of a Moor studded with emeralds and a 648-carat sapphire gifted by Tsar Peter I of Russia. It is unknown whether either of these were stolen.

Other valuable items include a jewel-studded sculpture of an Indian royal court, made out of gold, silver, enamel, precious stones and pearls.

Another is a 1701 golden coffee service by court jeweller Johann Melchior Dinglinger, decorated with lounging cherubs.

In 2010, then-museum director Martin Roth boasted in an interview with Die Welt that the Green Vault was ‘as secure as Fort Knox’.

Roth explained how the vault was protected by ‘invisible’ security systems, but warned that the biggest danger was information leaking out from inside.

The collection dates back to 1723, while the Dresden royal palace which houses it was first built in 1533 as the home for the electors and later kings of Saxony.

The Green Vault gets its name from the green-coloured columns and decoration in some of the rooms