Nobel Peace laureate Jody Williams told The Associated Press the letter has so far been signed by seven more awardees, including Mairead Maguire, Rigoberta Menchu and Shirin Ebadi.
On Saturday tens of thousands of demonstrators, many dressed in white, hit the streets of Madrid and other cities across Spain to demand dialogue to end the dispute.
The ideal would be not to have to take drastic measures.
Williams said the letter would be posted later Monday on the Nobel Women's Initiative website.
Caixabank, Spain's number 3 bank, and Banco Sabadell, the number 5, have both moved their head offices out of Catalonia last week following an independence referendum that the Madrid government attempted to block.
Spain's opposition Socialist party chief Pedro Sanchez called on Catalan leader Carles Puigdemont to "stop everything" as he threatens to formally declare independence within days. It said its priority was to protect "clients, shareholders and employees" given the current political and social situation.
Meanwhile, businesses have continued to announce their departure from the Catalan region amid the ongoing political uncertainty.
"If the Catalan government seeks to declare UDI [unilateral declaration of independence], I do not believe it would help their case now, and I hope they hold this back". Loiseau said the crisis following the 1 October referendum had to be resolved through dialogue within Spain.
Catalonia's Parliament had been due to meet on Monday with the expectation that it would endorse the declaration of independence.
'Tonight, we go to the World Cup'
England's Raheem Sterling (in white) battles for the ball with Slovenia's Bojan Jokic during the Group F qualifier at Wembley Stadium .
Puigdemont, a former journalist and not a career politician, said he is not afraid of going to jail over independence.
The French minister also noted that the recognition of Catalonia's independence would automatically mean its automatic exit from the European Union.
In that poll, deemed illegal by Madrid, 90 percent voted to break with Spain, but the turnout was well under half of the electorate.
The regional Mossos d'Esquadra police force, whose hierarchy reports to the Catalan government, had been in charge until now of guarding the palace in central Barcelona that hosts the judiciary.
Dyfrig Siencyn, who also represents Dolgellau on the council, has slammed the Spanish Government for abusing Catalonion citizens as they exercised their right to campaign for an independent state. "We will do everything that legislation allows us to ensure this", Rajoy told the newspaper.
But Catalonia's President Carles Puigdemont has pledged to push ahead for independence anyway.
Spanish Prime Minister Rajoy has not ruled out removing Catalonia's government and calling fresh regional elections himself if the region claims independence.
Earlier on Monday, First Minister Nicola Sturgeon said: "I didn't ever really take kindly to people trying to interfere in whether Scotland should become independent; it's not for me to say whether Catalonia should be independent".
To circumvent that ruling the Catalan President will now address the local Parliament on his own on Tuesday.