European stocks booked a fifth consecutive decline on Tuesday as heightened "Brexit" worries unsettled equity investors.

Polls: European stocks hit intraday lows Tuesday afternoon and losses sharpened into the close of trading as Reuters reported that the European Central Bank is preparing to publicly say it will support the financial markets, along with the Bank of England, if the U.K. votes in favor of leaving the EU. A referendum on the issue is slated for June 23, with the risk of an U.K.-EU breakup known as Brexit.

Investors have been seeking shelter in the bond market ahead of the referendum. Polls released Tuesday and Monday suggested that voters were in favor of leaving the EU. Among the surveys, market research firm TNS said 47% of likely voters back a Brexit, compared with 40% who are opposed.

A Sunday Times/YouGov poll showed 46% of Britons support leaving the EU, while 39% wish to remain.

U.S. stocks fell for a fourth straight session on Tuesday as the Federal Reserve kicked off its two-day policy meeting and fears over the U.K. voting to leave the European Union amped up. The Dow Jones Industrial Average DJIA, -0.33% fell 57.66 points, or 0.3%, to close at 17,674.82, as shares of American Express Co. AXP, -4.08% dropped more than 4%. The S&P 500 Index SPX, -0.18% declined 3.74 points, or 0.2%, to finish at 2,075.32, weighed down by the financials and materials sectors. The Nasdaq Composite Index COMP, -0.10% slipped 4.89 points, or 0.1%, to close at 4,843.55. Meanwhile, the CBOE Volatility Index VIX, -2.24% or so-called fear index, remained above 20.

Asian shares were slightly weaker but recovered from near three-week lows on Wednesday as markets digested U.S. index provider MSCI's decision not to include domestic Chinese equities in its indexes and Brexit fears drove investors to assets such as U.S. bonds and the yen.

Mainland Chinese shares, among Asia's worst performers this year, were mixed while Hong Kong slid, as markets, which had expected Chinese A-shares to be included in the emerging market index, considered the announcement.

MSCI's broadest index of Asia-Pacific shares outside Japan were down 0.1 percent. Japan's Nikkei reversed earlier losses to rise 0.7 percent.

MSCI in its decision said Beijing had more work to do in liberalising capital markets, and said it wanted more time to assess the effectiveness of the Qualified Foreign Institutional Investor (QFII) quota allocation scheme and capital mobility policy changes.

MSCI said it would consider China A shares' inclusion as part of its 2017 review and didn't rule out a potential off-cycle announcement should further positive developments occur ahead of June 2017.