(Updates with additional analyst comment)

By Kevin Buckland

TOKYO, May 20 (Reuters) - The longest-dated Japanese

government bond yields soared to all-time highs on Tuesday, as

investors' worries about demand for the debts flared following a

poor auction of 20-year securities.

Super-long JGB yields have been rising for weeks against a

backdrop of elevated U.S. Treasury yields and worries about

fiscal stimulus ahead of a crucial upper house election slated

for July, in a headache for the Bank of Japan that may soon be

compelled to act.

The central bank has steadily been reducing support for the

market by paring its monthly bond purchases, unwinding decades

of aggressive monetary stimulus.

Several parties have been calling for consumption tax cuts,

which Prime Minister Shigeru Ishiba has so far resisted.

On Monday, he told parliament that Japan's fiscal situation

was worse than Greece's at the height of the European debt

crisis, according to local media reports.

The lack of buyers at the Ministry of Finance's sale of

20-year JGBs provided a catalyst for a rush to the exits on

Tuesday.

"Investors' demand for duration is structurally slowing

down," JPMorgan analysts said in a research note.

"While price formation without intervention from the MoF and

the BOJ is ideal, some form of action is needed to stop the

collapse of super-long JGBs at present, or there could be

further super-long bond shocks triggered by downgrades or

additional fiscal measures."

The 20-year JGB yield rose as much as 15

basis points (bps) to 2.555%, the highest since October 2000,

after the finance ministry announced the auction results in the

early Tokyo afternoon.

The 30-year JGB yield soared 17 bps to a

record high of 3.14%, while the 40-year yield

surged 15 bps to 3.6%, also an all-time high.

The 10-year JGB yield climbed as much as 4.5

bps to 1.525%, its highest level since March 28.

Mizuho strategist Shoki Omori called the auction results

"lacklustre", "highlighting persistent supply-demand softness in

the super-long sector and fuelling concerns over who, if anyone,

will step in to buy."

Brokers and investors "appear reluctant to hold inventory,

raising the likelihood of a sell-off spiral that extends beyond

the 20-year tenor into both the 10-year and 30-year markets,"

Omori said.

The five-year yield rose 2 bps to 1.015%, a

level last seen on April 2, when U.S. President Donald Trump

announced his "Liberation Day" tariffs.

The two-year JGB yield touched 0.73%, the

highest since April 3.

Benchmark 10-year JGB futures fell as much as 0.47

yen to 138.78 yen, the lowest since April 2. Bond yields move

inversely to prices.

(Reporting by Kevin Buckland; Editing by Savio D'Souza and

Varun H K)