The Royal Pavilion estate has had four exhibits worth tens of thousands of pounds stolen in two raids this summer, The Argus can reveal.
A historic silver plate, worth up to £10,000, and three gem-encrusted antique items of jewellery have been taken from this prime tourists spot during opening hours.
A security review has been launched after the thefts but the GMB Union, speaking on behalf of security staff, has said that job cuts have left the city's "crown jewels" at risk from theft and staff's safety in jeopardy.
The city council's said its huge collection of art and artifacts, spanning half-a-dozen sites, were safe.
A spokesman said: "Improvements to security in the Royal Pavilion have already been undertaken in light of the first theft.
"We are in the process of undertaking detailed security risk assessments looking at all aspects of security of our buildings, collections, visitors and staff.
"Existing systems including CCTV coverage will be covered.
"The importance of these assessments cannot be underestimated in the light of this latest incident."
GMB Mark Turner said: "We are worried about the safety of our staff and protecting the city's crown jewels."
The 12-inch silver serving dish, dated 1807, was taken from Brighton's royal palace in broad daylight between June 1 and 3. It is not original to the Pavilion.
The affair was kept closely under wraps until The Argus uncovered details of the theft last week.
And on Friday, August 17, two Art Nouveau hair clips and a gold pendant were stolen at 11am from a locked draw in Brighton Museum.
One of the items, which were all created around the 1900s, is a French hair ornament set with diamonds by Louis Gautrait, who worked as principal engraver to Gariod, a leading firm of Parisian goldsmiths.
The stolen piece is one of his most elaborate and is fashioned in gold, with rose diamonds and borded with translucent plique-a-jour enamel.
A gold and plique-a-jour enamel pendant by Gautrait was recently valued at £8,000.
The two other stolen accessories are a Bohemian dragonfly made of silver gilt, opals and quartz and a pendant in gold set with two child baby teeth.
Mr Turner said his union had been raising concerns about staffing and the safety of the city's artifacts for a number of months.
He added he was concerned the review of the service could spell more cuts.
The information and security team guard a series of venues across the city including the Royal Pavilion, Brighton Museum and Preston Manor.
But the department was squeezed 18 months ago down by eight to 28, according to Mr Turner.
He said he had been in talks with members and the GMB were considering balloting for strike because of staffing levels.
He said: "We will be looking at a ballot and will looking at registering a dispute with the council.
"We have told them that we want more staff because staff levels are not right, particularly at locking up times."
The Pavilion and the city museums is home to fine art, antiques, textiles and other artifacts.
An important collection of Dutch 17th century paintings and 19th century British landscapes is among the collection, and many of the items are on loan to the council.
A council spokesman said: "Police investigations into both thefts are ongoing.
"We have briefed and will continue to brief our staff and union representatives on these risk assessments and are committed to involving them in the review process."
Police are appealing for witnesses to both thefts. If you have any information contact PC Vicki Lank on 0845 6070999.