Mr Monti also said that his Foreign Minister Giulio Terzi had resigned with motives not just limited to the marines' issue.
The premier, who himself was sworn in to replace Mr Terzi as interim foreign minister, gave more details behind the tangled diplomatic row, which Mr Monti said risked ruining relations with key trade allies in the developing world but denied that economics was a factor in the decision-making process.
Mr Monti said he was "stunned" by Mr Terzi's decision to step down, adding that his former chief diplomat gave no warning he would quit on Tuesday, and that his real aim was "to achieve another end that may become clearer in the near future," avoiding a more direct accusation, Italian news agency INSA said.
On Tuesday, Mr Terzi abruptly resigned claiming that his voice went "unheard" as the caretaker cabinet decided to send back the two marines - Massimiliano Latorre and Salvatore Girone - 10 days after it reneged on a pledge to return the two men who had been granted a four-week leave.
Mr Monti, addressing both Houses of Parliament last night, stressed on the diplomatic stakes. "There were serious and objective risks that Italy would find itself isolated in the international community," if it had failed to ensure the pair returned to India, Mr Monti said. "It would have opened a crisis of serious proportions with India".
Mr Monti said that since the pair had been returned relations had improved between the two countries that should lead to "a quick solution".
In his address to the House, Mr Monti insisted that economic interests did not influence the decision.
"Let me reject forcefully any speculation about possible exchanges or agreements with India," Mr Monti said.
Some media outlets have linked the marines' case to corruption allegations surrounding a Rs. 3,600 crore VVIP helicopter deal that is being probed by the CBI for alleged corruption.
Speaking before the Senate, Mr Monti said that Italy had faced "measures" from India and fellow BRICS countries as a consequence for choosing not to return the marines to face trial.
"We had word from Foreign Undersecretary Staffan De Mistura that the possibility of measures being taken against Italy were being considered," Mr Monti said.