* BNP Paribas takes top spot in bond syndications
* Some bankers say shift is because countries 'rotate' banks
* New rules, Brexit uncertainty have also brought change
By Abhinav Ramnarayan, Lawrence White and Maya Nikolaeva
LONDON/PARIS, Dec 7 (Reuters) - British bank Barclays has slid down the rankings in arranging European government bond sales as new regulations and Brexit uncertainty cause upheaval in the sector. Barclays had the largest share of the government syndicated bond market in 2015 and 2016 with over 10 percent, Thomson Reuters data show. That dropped to 7.3 percent of the 147.5 billion euros ($173.81 billion) market so far this year, putting Barclays in fifth position. BNP Paribas, which has been making a push into the market, took the top spot with a 9.3 percent share, the same as last year and up from 5.9 percent in 2015. U.S. lender Citigroup placed second with 9.2 percent, British bank HSBC followed with 7.8 percent and French bank Societe Generale took fourth spot with an increased market share of 7.4 percent.
A spokeswoman for Barclays declined to comment on why the bank had fallen in the rankings. Some of the higher profile transactions Barclays has been involved in include Spain's 8 billion euro sale of 10-year bonds and Italy's 6 billion euro 15-year deal.
But four countries that previously hired Barclays for syndicated bond deals in 2016 did not choose the British lender again to sell their bonds this year. These were Austria, Cyprus, Portugal and Slovakia.
Meanwhile, BNP Paribas and Societe Generale worked on France's high-profile 7 billion 20-year green bond and the latter on Austria's 3.5 billion euros 100-year bonds.
Citigroup did well in peripheral Europe. The U.S. bank was on two Spanish bonds, a 30-year Italian bond and a 10-year bond that was Belgium's biggest ever deal.
Bankers could not say exactly why Barclays' position had dropped, but two who work in European government bond sales told Reuters it could be accounted for by 'rotation'. Government debt officers regularly change the banks they use to help sell bonds after a number of years.
Others said it could in part be accounted for by Barclays' rivals having a stronger performance in supporting those countries' bond auctions.
The Barclays spokeswoman declined to comment on its performance in bond auctions.
European governments conduct all of their national borrowing through auctions or directly to investors in bond sales underwritten by banks, a process known as a syndication.
They hire banks to act as 'primary dealers', who buy bonds from them in auctions and distribute them on to investors. Successful primary dealers are rewarded by being selected to run syndicated debt sales which earn fees for the banks. Many banks make most of their profits in this sector from syndications.
Countries have different criteria for primary dealers, but they generally consider them to be successful if they have bought a large amount of debt at auction and then sold it to clients to create a liquid secondary market which reduces borrowing costs.
Another factor in Barclays ratings fall could be governments' desire to strengthen ties with banks already based in the euro zone ahead of Britain's departure from the European Union, some bankers said.
"No one knows what the landscape is going to look like after Brexit, but what we are seeing in general is a lot of clients coming to us because of our connections in Brussels, given the uncertainty," said a banker at one of the French banks, who declined to be identified speaking about his clients.
Bankers said having a strong presence in the eurozone was becoming important for winning mandates as EU officials are considering imposing rules to require banks that are primary dealers to have significant operations in the EU post-Brexit.
British bank HSBC said that its large French subsidiary helped it compete against rivals in the sovereign debt business.
"Brexit is neutral for us, to positive versus our peers, in terms of our ability to compete in the Euro sovereign bond space, because our French bank is already the legal entity for all our Eurozone sovereign dealing so nothing will change for us," said Frederic Gabizon, head of European Public Sector at HSBC.
Barclays operates the bulk of its European business out of London. It is in talks with regulators to make its Irish unit its EU hub after Brexit.
While Brexit is likely to bring new regulatory challenges, several banks had already pulled out of the European government bonds business in recent years to meet the requirements of other new rules requiring them to hold capital to offset potential losses from their trading business.
Credit Suisse cut all its European primary dealer relationships apart from Greece in 2015.
The move was part of a drive by the Swiss lender to scale back operations in Europe and Asia after announcing plans to boost its capital to meet stricter Swiss financial regulations on balance sheets.
Since then, other banks such as Deutsche Bank, Credit Agricole, ING, Commerzbank, RBC, Scotiabank and Santander are among those that have either withdrawn or been told by individual governments that they have not met their criteria for being a primary dealer.
"With numerous regulations either due to be, or already implemented impacting banks' balance sheets, there will no doubt be an element of asset deleveraging, this will be reflected in banks' strategic decisions to withdraw or expand in certain market segments," said Victoria Webster, director of fixed income at the Association for Financial Markets in Europe (AFME). ($1 = 0.8486 euros)
Editing by Anna Willard