Serie A president Maurizio Beretta said he would not be moved by threats after the captains, including Inter Milan's Javier Zanetti and AC Milan's Gennaro Gattuso, said the championship would not start without the agreement being signed.
"The Serie A players would like to inform the public of the worrying situation concerning the renewal of the collective bargaining agreement," said the letter.
"The previous agreement which was certified by the Italian federation (FIGC) in December 2010, was ignored by the clubs, which was an unacceptable decision.
"As a result, the period of de-regulation continues to go on. The current situation threatens the players' rights.
"Italy is today the major only football nation without contractual rules for the players and it is time to solve this situation.
"We reiterate that it will not be possible to start a new championship without the signing of the agreement."
Last week, Italian Players Union (AIC) president Damiano Tommasi had also warned that the new agreement had to be signed before August 27, when the league is due to get under way.
An old collective agreement expired at the end of the 2009/10 season and talks for its renewal dragged on for the whole of last season, when strikes were twice narrowly averted.
The union is especially unhappy about clubs trying to force players to move in the last year of their contracts and players who are no longer wanted by their clubs being forced to train separately.
Beretta told the ANSA news agency on Sunday: "I'm surprised. We will not sign under these conditions and I don't see why we should submit to a threat from Tommasi, something we didn't accept with [predecessor] Sergio Campana.
"The strike threat by the players is a serious and insensitive act, especially considering the general situation in the country.
"We mustn't forget that we are talking about 800 players whose average wage is over one million euros per year."
