Mexico’s central bank could appeal to the country’s highest court if rules requiring it to absorb excess dollars in the economy are approved, a move that bankers and critics say would undermine Banxico’s autonomy, put its reserves at risk and force it to launder illicit drug cartel cash.

The bill was approved this week in the Senate and is now before the lower house of Congress. It could be put to a vote as early as Monday.

Ricardo Monreal, a senior figure in populist President Andrés Manuel López Obrador’s Morena party who presented the bill, argued that it was a “social” reform to help migrants with dollars in cash or those who receive greenbacks in the restaurant and tourist trades.

But Alejandro Díaz de León, Banxico governor, told legislators on Friday it would benefit just one bank which has recently had problems exporting dollars, and was “not a generalised problem”.

He did not name the bank but Emilio Álvarez Icaza, an independent senator, this week tweeted that billionaire businessman Ricardo Salinas’ Grupo Azteca banking, retail and media conglomerate “lobbied to get [it] approved”.

Mr Díaz de León has not ruled out legal action if the bill passes the Chamber of Deputies. Asked in an interview with Radio Fórmula if the central bank could seek an injunction at the Supreme Court, he said: “That alternative is open.”

Under current rules, US dollars received in Mexico are changed into pesos. Any that are not used are repatriated to the US through correspondent banks, which act as intermediaries, or sometimes sent to Canada and Spain. According to the central bank, banks repatriated $4.7bn this way in the first nine months of 2020.

Increasingly tight anti-money laundering regulations in recent years, however, have clamped down on correspondent banks willing to do business with Mexican institutions. Banks were left with $102m in the coffers that they were unable to send abroad during the first nine months of the year, the central bank said.

The new law would force Banxico to buy those dollars and add them to its own reserves, which would affect both its balance sheet and its autonomy to take measures it considers best to control inflation.

Gerardo Esquivel, one of the members of Banxico’s board appointed by Mr López Obrador, expressed worry in a tweet this week about the risks to the central bank’s international reserves if it was compelled to hold illegally obtained funds.

Mr Díaz de León has warned that in money laundering investigations, foreign authorities can freeze accounts.

Gabriela Siller, head of economic and financial research at Banco Base, said: “It could have its reserves frozen — the risks [for the central bank] are far more serious. It’s crazy to be putting Banxico in this situation.”

Mr Díaz de León told Friday’s hearing that in the first nine months this year “one [bank] stands out with an upward trend in the accumulation of surplus [dollars]. In fact, only one institution has accumulated more than $10m . . . This shows that the accumulation of excess dollars that have not been able to be exported is a particular problem for one institution and is relatively recent.”

He said 15 Mexican banks had exported dollars using 10 correspondent banks.

Mr Salinas, a close adviser of the president, defended the bill on his blog, saying that suggestions it would imperil Banxico’s autonomy and open the door to buying dollars from cartels were “categorically false and alarmist”. His group had no comment on whether it had lobbied senators.

Coparmex, an employers’ confederation opposed to many of Mr López Obrador’s policies, said the damage from the reform could be “irreparable”.

Ms Siller also feared that this bill could open the door to future changes in how Banxico can spend reserves.

Rocío Nahle, energy minister, was reported to have recently considered it an “excellent” idea to use them to prop up Pemex, Mexico’s struggling state oil company. Argentina’s government in 2010 allowed its central bank to use reserves to pay off debt, something currently barred in Mexico.

Despite the outcry, Mr López Obrador this week appeared to close the door to any U-turn on the bill, saying that if it were approved by legislators “they are reforms that must be respected”.