The banks, which have already made huge provisions against other exposure, will be forced to make even more of them against their Dubai debt, which will cut deep into their profits and may lead to losses, they added.
"The crisis of the Gulf financial sector, especially banks, will further deepen. In my opinion the exposure will be too huge and the consequences highly negative," Kuwaiti economist Hajjaj Bukhdur told AFP.
The main impact will obviously be on the banks in the United Arab Emirates, especially Dubai, but other Gulf institutions will be hard hit, he said.
Bukhdur estimated the exposure of Gulf financial institutions to Dubai debt to be "in the billions of dollars," for which they must make provisions.
State-owned Dubai World surprised the world on Wednesday by announcing that it wants to delay payment of its debt for at least six month until May next year.
The government conglomerate is estimated to owe 59 billion dollars out of Dubai's debt of 80 billion dollars, while a 3.
5-billion-dollar (2.9 billion euro) Islamic loan to its real estate arm Nakheel matures on December 14.
The involvement of Gulf states in Dubai also comes through huge investments in the emirate's real estate and leisure sectors, which have been badly affected by the global financial crisis. Banks in the Gulf have already been hit by the default of some Kuwaiti investment firms on debt estimated at around 10 billion dollars.
They also had exposure to defaults on 22 billion dollars in debt by the Saudi Saad and Al-Gossaibi business groups.
Following the Dubai World announcement, the Bahrain-based Gulf International Bank decided to postpone its first transaction of a four-billion-dollar bonds issue.
Moody's Investor Services took no immediate rating action on any of the UAE banks, but added in a statement late on Friday that it was reviewing the positions of 17 UAE banks and five investment firms.
Moody's said it will not "implement any rating changes for now given that the banks that have sizeable exposures to Dubai World are either already on review for possible downgrade or carry a negative outlook on their deposit ratings."
"Dubai-based banks are generally more heavily exposed to Dubai World and Nakheel than their counterparts in Abu Dhabi, with the exception of Abu Dhabi Commercial Bank and First Gulf Bank, whose ratings carry negative outlooks," it said.
"Moody's has no reason to believe that the UAE's federal government would abstain from supporting banks in Dubai or in other emirates," the agency added.
Standard and Poor's has placed four Dubai banks on its creditwatch with negative implications because of their large exposure to Dubai World and Nakheel in addition to other government-related entities.
Saudi economist Abdulwahab Abu-Dahesh said he believes Saudi banks will not be directly affected by the Dubai decision, though he added that some of these banks have bought Dubai government bonds.
"I believe that profits of many banks of the region will be negatively affected.
Some will end up 2009 in the red because of the provisions they must take," Abu-Dahesh told AFP.
"Its a very serious and severe problem that is likely to shake up the Gulf financial system as a whole.
I expect Gulf bourses to dive like the September crash last year," following Lehman Brothers bankruptcy, he said.
Standard and Poor's has said Dubai and its companies are due to repay around 50 billion dollars in debt over the next three years.
Bukhdur believes the impact of the Dubai debt crisis will be felt more by Gulf banks in 2010.
"The impact on Gulf banks will be much stronger in 2010. I believe that after UAE, banks in Saudi Arabia will be the most affected followed by banks in Qatar and Kuwait," he said.
Monica Malik of EFG-Hermes investment bank said the Dubai crisis is likely to tarnish confidence in the Gulf region.
"The loss of confidence externally towards Dubai will be marked and should not be underestimated, but is also likely to impact the wider region, although we reiterate marked difference in economic realities on the ground," she said.