S Alam Group owner Mohammed Saiful Alam claimed an international investment pact and his status as a Singaporean citizen offer protection against a "campaign of intimidation" being waged by the country's central bank against his conglomerate, according to a report by Financial Times (FT).

In a letter sent by lawyers on their behalf, Saiful Alam, founder and chairman of industrial conglomerate S Alam Group, and members of his family warned central bank governor Ahsan H Mansur that they may seek to begin international arbitration procedures against Bangladesh.

FT said the letter from law firm Quinn Emanuel Urquhart & Sullivan follows Mansur's allegations in an interview with the Financial Times that Saiful Alam and associates "siphoned off" at least Tk 1.2 trillion (around $10 billion) from Bangladeshi banks during the regime of ousted former prime minister Sheikh Hasina, it added.

The letter, a copy of which was seen by the FT, accused Mansur of making "inflammatory and unsubstantiated public comments" amounting to a "campaign of intimidation against a business group" that it said employs about 200,000 people directly and indirectly in Bangladesh.

The letter and Saiful Alam's threat to pursue international arbitration mark his most serious pushback yet against Bangladesh's interim government, which took power after a student-led protest movement toppled Sheikh Hasina.

In an interview with the Financial Times last month, Mansur — who was appointed Bangladesh Bank governor after Hasina fled the country in August — said S Alam and his associates had "siphoned off" at least $10 billion "as a minimum" from the banking system after taking control of banks with help from the Directorate General of Forces Intelligence.

Mansur alleged Saiful Alam and his associates took out loans from banks and inflated import invoices to carry out the "biggest, highest robbing of banks by any international standards".

The S Alam Group, which has interests in sectors including food, construction, garments, and banking, rejected Mansur's allegations last month, saying in a statement through Quinn Emanuel that there was "no truth" to the statements.

FT said the letter to Mansur — sent on behalf of Saiful Alam, his wife Farzana Parveen and his sons Ashraful Alam and Asadul Alam Mahir, who it said collectively owned and controlled a "major part" of S Alam group — described the allegations as "deliberately false and defamatory".

"Your statements only further the aims of an apparent campaign designed to destroy the S Alam Group, and therefore also the investors' investments," it said.

"Notably, you appear to be leading, if not orchestrating, that campaign."

FT added that the letter said all four investors were Singaporean citizens. 

A spokesperson for Quinn Emanuel did not respond to a request for comment on when Alam's family had acquired Singaporean citizenship and whether they remained Bangladeshi nationals. 

The government of Singapore also did not respond to a request for comment.

Bangladesh and Singapore have a bilateral investment protection treaty dating back to 2004.

The letter said Mansur's statements as central bank governor were "attributable to the State of Bangladesh" under the pact. 

It added that as Singaporean citizens, the investors also enjoyed "the rights and protections" granted by a 1980 Bangladeshi law on foreign private investment.

The investors' letter said they would "take all necessary measures to protect their rights, including but not limited to" requesting arbitration by the International Centre for Settlement of Investment Disputes.

Mansur told the FT that the claims he made in the interview were "fully substantiated" when asked for a response to the letter. "They are still being documented because the range of corrupt activities are widespread across many banks and over many years," he added. "The full documentation will take some time."