* Greek referendum fears threaten bank financing window

* Bank shares tumble, CDS and bond spreads widen

* RBA cuts rates, Fed begins meeting, ECB eyed

By Kirsten Donovan

LONDON, Nov 1 (Reuters) - The brief financing opportunity that banks have enjoyed in recent weeks is set to end abruptly after the Greek prime minister's call for a referendum on the country's bailout deal raised fears of its uncontrolled default.

After a three-month lull, October produced a handful of senior unsecured bond deals from issuers such as Deutsche Bank, ABN Amro and BBVA, taking advantage of improved sentiment after the European Central Bank extended its support measures to financial institutions and politicians promised a comprehensive solution to the euro zone debt crisis.

But Greek Prime Minister George Papandreou's shock decision on Monday to hold a referendum thrust fears over the future of the euro zone back to the surface.

If Greek voters reject the unpopular bailout plan it could result in a hard default which could force banks to take even deeper losses on their Greek sovereign bonds than the 50 percent agreed last week.

"The financing window has slammed shut just as it was beginning to open," said Societe Generale credit strategist Suki Mann.

"And if the Greek uncertainty continues, then it will stay closed for the rest of the year."

Shares of French banks and other lenders exposed to Greece and other weak euro zone countries slumped, while the Markit iTraxx Senior Financial credit default swap index widened around 30 basis points to over 250 basis points.

The subordinated equivalent was over 50 basis points wider and an 18-month senior bond issued by Spanish bank BBVA on Friday was quoted 30 basis points wider since launch, with the deterioration in sentiment likely to keep both borrowers and lenders out of the market.

Senior financial issuance has totalled 118 billion euros ($165 billion) so far this year, compared with 150 billion euros in the previous two years and over 300 billion euros in 2006 and 2007, according to SG.

"If the market does settle down you could get the big names come in but there isn't going to be a flood of issuance," Mann said.

"We'll be lucky to see another 5 or 10 billion euros worth by year-end."

Banks have around 320 billion euros of senior financing maturing next year, while many are facing increased capital requirements as a result of last week's proposed EU rescue plan.

"Considering the funding difficulties that banks have faced ... we think that issuers that are capable of doing so will opt for issuance further out the curve to reduce their refinance risks," said ING credit strategist Maureen Schuller.

"However, the group of banks that can do so will be rather limited."

The shift to shorter-dated debt has been notable with 64 percent of non-covered financial supply issued this year having maturities of less than 4 years, according to ING, compared with 51 percent in 2010 and just 21 percent of issuance this year by corporates.

"In particular, weaker rated or peripheral financial institutions tend to issue more at the front end of the curve," Schuller said, adding that this also pushed spreads wider.

Signs of the renewed stress were also visible in money markets where the spread of three-month Libor rates over equivalent maturity overnight indexed swap rates widened 7 basis points to 79 basis points, back to within a basis point of its highest levels since the 2008 financial crisis.

CENTRAL BANKS

The Federal Reserve begins its two day policy meeting on Tuesday with officials expected to lean on a modest improvement in economic data to justify a wait-and-see policy approach.

Earlier, the Reserve Bank of Australia cut rates, prompting a rise in interest rate futures as investors priced in more easing in the months ahead.

The RBA cut its main cash rate by 25 basis points to 4.5 percent, the first easing since early 2009.

On Thursday the European Central Bank holds its first policy meeting under new head Mario Draghi just as the Greek referendum plan takes the euro crisis to a new level.

Despite that and sluggish growth, markets were not expecting a rate cut even before data on Monday showed euro zone inflation holding at 3.0 percent last month, way above the ECB's target of close to but below 2.0 percent.