A woman suffering flu-like symptoms gave birth standing and fully clothed while detained near the Mexican-US border, according to the American Civil Liberties Union.

The Guatemalan woman, 27, was being processed at the Chula Vista Border Patrol Station near San Diego when her complaints of pain and pleas for help were allegedly ignored by agents, according to a complaint filed on Wednesday by the ACLU and Jewish Family Service of San Diego with the US Department of Homeland Security's Office of the Inspector General.

The woman, holding onto a garbage can for support, was repeatedly told to sit and wait to be processed. Her husband, hearing a baby's cries, removed her pants to reveal the newborn's head while their daughters, aged 2 and 12, witnessed the event, according to the ACLU's interviews with the family.

"This horrific case is just the most recent and one of the most egregious examples of this agency's abuse," said ACLU attorney Monika Y. Langarica.

An account of the birth by US Customs and Border Protection (CBP), released a few days after the incident on 16 February, describes a conflicting version of events.

After entering the country illegally, the family was detained but "did not appear to be in distress and did not request any medical attention", the agency said in a statement.

"Thanks to the medical resources available in our stations, this woman and her child were well cared for and received immediate medical attention," said Chief Patrol Agent Aaron Heitke. "Our agents are well trained to manage the unexpected, and I'm proud of the work they did in caring for this mother."

A CBP spokesman told The Independent that after an internal investigation of the complaint, the agency "strongly disagrees with the unsubstantiated allegations against our agents".

"Based on this available information, CBP supports what appear to be nothing short of heroic actions of medical personnel and agents on scene and welcomes the response of [Department of Homeland Security Office of Inspector General]," the spokesman said.

In a letter to Department of Homeland Security Inspector General Joseph Cuffari, Democratic Senator Richard Blumenthal demanded an investigation into the incident, as well as other reported mistreatment of pregnant women in custody.

In the letter, Mr Blumenthal said the woman was not allowed to shower after giving birth and after returning from hospital to the Border Patrol station two days later was refused a blanket for the baby.