A 999 call from a woman who discovered her three-week-old son had stopped breathing went unanswered because there were not enough ambulance staff on duty, an inquest has heard.

Lauren Braisby, 17, was put on hold while the operator tried twice on two different phone lines to contact South East Coast Ambulance Service’s control room in a bid to get paramedics to her son Jenson, East Sussex Coroner’s Court was told on Thursday.

Senior coroner Alan Craze found the baby died from natural causes, but sought assurances from the ambulance trust that improvements to the way it handled calls were being made.

A South East Coast Ambulance Service NHS Foundation Trust (Secamb) spokesman said in a statement that it had carried out a “thorough investigation” into the incident and “significant improvements” to the way calls were answered had been made.

He added: “Our thoughts are with the family at this difficult time and we are very sorry for the service they received in such tragic circumstances.”

On August 25, 2017, the mother woke at around 4am to feed identical twins Alfie and Jenson.

However, after attending to Alfie, she found Jenson unresponsive so dialled 999, the hearing in Eastbourne was told.

A BT operator tried to connect the call but put her on hold when no-one answered and tried the back-up critical phone line, which is used for urgent incidents when the control room is busy.

The inquest heard that calls of this nature should be placed in the highest category with an ambulance responding within seven or eight minutes.

When the second attempt to connect to the control room went unanswered, Braisby, who lives in Polegate, hung up and the family decided to drive the baby to hospital.

They instead managed to get help by flagging down a passing ambulance which was on its way to another incident.

Paramedic James Punchard told the inquest Jenson was “lifeless and not breathing”, adding: “When being handed the baby it was obvious he was in cardiac arrest.”

The team began life support and rushed him to hospital but he was pronounced dead shortly afterwards.

Secamb was put into special measures in 2016 after the Care Quality Commission (CQC) watchdog rated it inadequate.

An inspection found it was not meeting national response time targets and had “weak” safeguarding arrangements.

Low staffing levels were causing fatigue and affecting performance, while procedures were not being followed, the report said.

The government handed Secamb a £10m grant to improve its service and hire more staff, the inquest heard.

In November last year, the CQC welcomed improvements made so far by the trust but said more work needed to be done; it is now rated as requiring improvement.

A serious incident report into Jenson’s death found there were not enough staff on duty in the control room to meet the number of incoming emergency calls and there was no written procedure for when calls were transferred to the critical line, consultant paramedic Michael Bradfield told the inquest.

The trust had also been in the middle of rolling out a new computer system at the time.

Although born prematurely on July 4 aged 31 weeks and weighing just 3lb 15oz (1.8kg), Jenson had been recovering well after spending a week in an incubator, the inquest heard.

Craze told the court that he thought the baby may not have been breathing for some time.

Detective Sergeant Julie Sapwell said the death was not suspicious, but raised concerns about the length of time it took the ambulance service to notify police of the incident, delaying the start of their investigation.

Bradfield told the inquest that the “unusual” manner in which the incident unfolded meant the routine call to police was not made and paramedics were unable to record full details of the incident in order to pass this information to the police and refer the case to a safeguarding team.

Mark Bailey, operations unit manager, told the court: “There were potential points of failure in that process” but insisted improvements were now “95%” complete.

The inquest heard that staffing had been a “serious issue” for Secamb and there were “high levels” of staff on long and short-term sick leave.

Bailey added: “At the time sickness was very high because of the pressure on staff.”

He said a recruitment drive would see growing numbers of staff join in the next couple of months and other improvements should be in place by June.

Ruling the death was due to natural causes from sudden infant death syndrome (Sids), also known as a cot death, Craze said he was “satisfied” the trust was making progress in its plan to improve the way it operates and prevent similar incidents in the future.

He told the hearing he was “sad” that the court had been unsuccessful in its attempts to involve the parents in the inquest process, adding: “The other twin is extremely ill and is taking up all of the mother’s time in being cared for.”