Gulf Arab investors, some of the biggest buyers of British real estate, are holding back from new deals because they fear a property price slump if Britain leaves the European Union, according to legal and investment sources. Sovereign and private investors from Qatar, Saudi Arabia, Kuwait and the United Arab Emirates have been prolific buyers of British assets in the past decade, snapping up billions of dollars worth of property, mostly in London.The value of residential property in upmarket areas popular among Gulf investors - including Chelsea, South Kensington and Knightsbridge - fell between 3.5 and 7.5 percent on the year in May, according to estate agent Knight Frank. Gulf family businesses and private investors are heavily involved in London real estate. Investors from the UAE accounted for more than 20 percent of buy-to-let property sales in the U.K. in 2015, said Amit Seth, the Middle East and North Africa head of international residential developments at London-focused real estate agency Chestertons. While the precise impact on Gulf investments is unclear, overall flows of foreign capital into commercial real estate in Britain stopped in the first three months of 2016, Bank of England Governor Mark Carney said in April. Business investment in the country also fell in early 2016, statistics showed this week.Gulf investors also have broader worries about their investments in other sectors and how a possible Brexit in a June 23 EU referendum could affect the British economy, the sources said. Sheikh Hamad Bin Jassim Bin Jaber al-Thani, a former Qatari prime minister and investment chief who oversaw much of the Gulf state's UK acquisitions, has spoken out against a "leave" vote."In the Middle East we all want to see a strong Europe, and believe that economic integration is key to making it stronger. In fact, we believe the U.K. should not only be part of the E.U. but should lead it," he told Reuters, describing the City of London as the "financial capital of the world". Qatar is one of the most high-profile investors in London, owning landmarks such as the Shard skyscraper, Harrods department store and Olympic Village, as well as luxury hotels.It also leads a consortium that bought the owner of the Canary Wharf financial district last year.While the Qatar Investment Authority (QIA) wealth fund has been diversifying its portfolio away from Europe towards more investments in the United States and Asia in the last couple of years, it is still heavily invested in Britain and holds stakes in Barclays, Royal Dutch Shell and Sainsbury's. If Britain votes to leave "then you are going to see a big hit to investments", said a senior Qatari banker who does business with sovereign and private investors. He said investors were still working on deals without finalising them until the picture becomes clear. "They are watching to see what happens, but people are continuing to work on new things as they take months to get done."