Shares in the Bank of Japan (BOJ) and the Swiss National Bank (SNB) gained as much as 21% and 4.6% respectively, but the absence of any fundamental drivers suggested retail investors were piling in.

Both central banks set monetary policy in economies with the world's most popular funding currencies. Their peculiar ownership structures and low free float can exacerbate moves in their share prices.

Shareholders' rights in both banks are also severely limited, which should theoretically narrow their investment appeal.

"One could see the rise (in BOJ shares) as symbolic of how markets are driven by excess liquidity," said Takashi Nakamura, a senior strategist at Japan's Tokai Tokyo Research Institute.

"We're starting to see signs of a bubble everywhere in the stock market," he added.

BOJ shares climbed above 40,000 yen ($374), their highest level since October 2018, having risen 46% over three sessions. The government owns more than half of the stock.

Veteran investors were reminded of the late 1980s when Japan was experiencing an asset-price bubble and BOJ shares became the target of speculative trading, rising 40-fold in four years.

SNB shares built on Monday's strong gains after the bank posted a net 2020 profit of 20.9 billion Swiss francs ($22.8 bln). At one point, the stock was up more than 10% in two days.

Traders in Zurich said retail investors were most likely driving the move but volumes remained thin.

The SNB declined to comment on its share price move.

UNORTHODOX

The last sharp rise in SNB shares was in April 2018, when they climbed from below 2,000 Swiss francs to a record 9,760 francs in less than a year. That surge came after blogs in Germany compared the SNB's shares to ultra-safe perpetual bonds.

Voting rights in SNB are subject to restrictions, with most of its shares held by the country's cantons and regional banks. The bank has been listed since its foundation in 1907, and 40% of its shares are held by the public.

SNB-watchers said the legal limit on dividends, at 6% of the share capital, also conflicted with the stock gains.

"A change in the SNB's dividend policy is not in the cards, fundamentals stay the same," said UBS economist Alessandro Bee.

The pullback in the Japanese yen and Swiss franc against the U.S. dollar this year, however, bodes well for the central banks as the value of hundreds of billions of dollars' worth of overseas assets on their balance sheets rises.

The SNB and BOJ are known for their large portfolio holdings. The SNB's 914 billion francs of foreign investments are bigger than the Swiss economy.

The Swiss bank is a significant investor in U.S. tech stocks such as Apple In and Tesla Inc, while the BOJ has huge holdings in the local share market, which recently scaled a 30-year high.

($1 = 106.9100 yen)

($1 = 0.9163 Swiss franc)

(Reporting by Danilo Masoni in Milan, Fumiya Mizuno and Leika Kihara in Tokyo, John Revill in Zurich and Aaron Saldanha in Bengaluru; Editing by Vidya Ranganathan and Matthew Lewis)

By Danilo Masoni and John Revill