* S&P cuts to AA minus, same as China and Saudi Arabia
* Says government has no "coherent" strategy to tackle debt
* Downgrade throws spotlight on heavily indebted industrialised world
* Japan economics minister says move regrettable (Adds details)
By Tetsushi Kajimoto
TOKYO, Jan 27 (Reuters) - Standard & Poor's cut Japan's credit rating on Thursday for the first time since 2002, saying Tokyo lacked a plan to deal with its mounting debt, in a warning that will rattle other heavily indebted rich nations.
The agency reduced Japan's long-term sovereign debt rating by one notch to AA minus, three levels below the highest possible rating. It said Japan's fast-ageing population, persistent deflation and the loss of the coalition's upper house majority had compounded the government's fiscal challenge.
Politicians and credit ratings agencies have been warning for years that Japan needs to lower its public debt, by far the worst among rich nations at double the size of its $5 trillion economy, but progress has proved elusive.
The ratings cut is a forceful reminder of the fragile financial state some rich nations are in following the global credit crisis.
The U.S. budget deficit is expected to blow out to a record $1.5 trillion this year and debt worries in Europe have already prompted financial rescues of Greece and Ireland.
Japanese Prime Minister Naoto Kan has made tax and social security reform top priorities and the S&P downgrade adds pressure on him to galvanise a divided parliament.
Julian Jessop, chief international economist at Capital Economics in London, warned of the consequences if Tokyo failed to get its fiscal house in order.
"If it looks like making a mess of this, further downgrades will surely follow. Given the size of Japan's economy and the current sensitivity of global financial markets to sovereign debt concerns, the impact would be felt worldwide," he said.
"It supports our fear that 2011 could be the year when Japan's dire fiscal position finally impacts markets both at home and internationally."
The yen and Japanese government bond prices fell and the credit default swaps spread on Japan widened after the announcement.
But markets in the past have not worried too much about the country's high debt because it is well serviced by ample domestic savings and few foreign investors hold Japanese government bonds (JGBs).
However, Japan's society is ageing quickly, so social welfare costs will take up an increasing proportion of the budget in the absence of reforms, which S&P said reduces Japan's already weak fiscal flexibility.
S&P's downgrade leaves its credit rating on Japan one notch below both Fitch and Moody's and on a par with countries including China and Saudi Arabia. The new level is one notch below Spain.
"The downgrade reflects our appraisal that Japan's government debt ratios -- already among the highest for rated sovereigns -- will continue to rise further than we envisaged before the global economic recession hit the country and will peak only in the mid-2020s," S&P said in a statement.
"In our opinion, the Democratic Party of Japan-led government lacks a coherent strategy to address these negative aspects of the country's debt dynamics, in part due to the coalition having lost its majority in the upper house of parliament last summer."
Analysts say a Japanese debt default is unlikely because of Japanese household assets of some 1,400 trillion yen, which at three times the size of economic output provide a healthy pool of savings to fund the borrowing.
"Japan's public finance problems are a long-fuse issue. The downgrade doesn't mean a crisis is imminent. It signals increased vulnerability," said Tim Condon, head of research in Asia for ING Financial Markets in Singapore.
"Foreigners don't buy Japanese government bonds so the crisis risk comes from Japan's death-spiral demographics. The downgrade is bad for G3 government debt because it spotlights their weak public finances."
REVERBERATES
Japan's debt has been growing for years as it tried to revive the economy after a property bubble burst in the early 1990s, while other developed countries are tackling massive public debt built up during the global financial crisis.
Debt markets have punished fiscally weak countries in Europe, which led to the bailouts of Greece and Ireland and even raised questions about the future of the euro currency. Fears of contagion have put both Portugal and Spain in the spotlight.
In European markets, Spanish, Irish and Italian bonds yields were higher on the day and the cost of insuring euro zone sovereign debt against default rose.
The ratings move on Japan also pushed credit default swaps on triple-A rated debt higher, with the spread on German CDS hitting its highest level since March 2009 at 63 bps.
"It's affecting countries with high debt burdens in the euro zone and anything that weakens the euro zone can have a knock-on effect on Germany, even though its finances are in much better shape," said Markit analyst Gavan Nolan.
The United States has been clinging on to its top AAA credit rating despite worries about the expanding budget deficit, which the Congressional Budget Office said this week would reach $1.48 trillion in fiscal 2011.
Japanese bonds showed a mild reaction to the S&P ratings cut, reflecting the market view that Japan is highly unlikely to default on its debt when its borrowings are easily funded.
The benchmark 10-year yield rose just 2 basis points to 1.250 percent. However, they could come under upward pressure, one trader said.
"The downgrade today was not surprising. Japan's rating has been constantly under threat over the past decade and many in the market had even taken these events to buy JGBs on price dips," the trader in Japan said.
"But we may no longer be able to be so complacent. Japan's finances have steadily worsened since the last downgrade under the government's seemingly tacit approval. Yields look to rise going forward, not down."
OPPORTUNITY
Fitch Ratings said Japan's rating was supported by its ability to fund itself, although a failure to make progress to reduce the fiscal burden could put pressure on its ratings.
Moody's Investors Service reiterated its ratings, a spokesman said. Both Fitch and Moody's have a stable outlook on Japan.
S&P warned a year ago it might cut Japan's credit rating unless it came up with a plan to deal with its debt.
Japan's outstanding long-term government debt is set to reach 869 trillion yen ($10.57 trillion) at the end of March this year, or 181 percent of gross domestic product (GDP), Japan's Ministry of Finance says.
If short-term debt is added, Japan's liabilities will hit 204 percent of GDP this calendar year, larger than 137 percent for Greece and 113 percent for Ireland, OECD figures show.
The government plans to issue a record 144.9 trillion yen in bonds in the fiscal year that starts on April 1.
After Thursday's cut, S&P said the outlook on the long-term rating was stable, reflecting its view that Japan's strong external balance sheet and monetary flexibility partially offset the pressures stemming from the fiscal side.
Japan's government is well aware of its debt problem but, like administrations before it, has struggled to tackle it head on. Just this month, Economics Minister Kaoru Yosano warned that the country faced a fiscal dead end. He said on Thursday the S&P move was regrettable.
Prime Minister Kan is pushing for a debate on increasing the national sales tax, which at 5 percent is among the lowest among major economies, that he says is vital to pay for huge welfare costs.
Kan's key economic ministers have promised to impose fiscal discipline, something Finance Minister Yoshihiko Noda reiterated in reaction to the S&P downgrade.
Still, the government is pr