The market, first granted a charter by Edward III in 1327, generates 200 million pounds ($300 million) in annual revenues and its porters transport 25,000 tonnes of fish every year.
But the City of London wants to revoke their licenses, saying a 19th century bylaw governing them is obsolete and does not guarantee jobs or standards of porters.
Their union, Unite, strongly objects.
"The proposals by the City of London Corporation will destroy this important London market," said Jennie Formby, national officer for Unite, in a statement.
"The changes will damage how the market functions and wreck an important part of London's heritage. The local community and London as a whole must now fight alongside these workers to ensure we can save Billingsgate Market."
The porters would not go on strike but campaign against the plan regularly with the local community.
The City of London said it was confident the market could "adapt to changing circumstances".
"The byelaw relevant to porters, dating from 1876, permits the Superintendent to license anyone of 'good character and fitness' to be a porter," it said in a statement.
"The license is, therefore, a permit to work but does not guarantee employment, nor does it guarantee standards, which are a matter for employers."
Their backers say the porters are highly trained and know how to ensure the smooth running of the market.
"I support the fish porters in their campaign to retain the market and its heritage," Ken Livingstone, former Mayor of London, was quoted as saying in the Unite statement.
"The City of London Corporation and the London Fish Merchants Association need to think again and reverse their proposal for the market."
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