A man who died at an immigration removal centre (IRC) expressed suicidal thoughts and had “inadequate” medical care, a report said.
Amir Siman-Tov, 41, was being detained at Colnbrook IRC when he said he overdosed on codeine and ibuprofen, but was tested for neither at hospital.
The report said he left hospital with “no clear discharge instructions” and died the next day on 17 February 2016.
The Home Office said recommendations from reports “are treated seriously”.
A report by the Prisons and Probation Ombudsman published earlier this week said Mr Siman-Tov, from Morocco, was given asylum in the UK in 2001 but was liable for deportation after serving a prison sentence.
He was detained at the centre near Heathrow Airport after breaching bail conditions on 25 January 2016 and transferred to Colnbrook IRC, where he told a consultant forensic psychiatrist he would “rather die than be deported to Morocco”.
On 16 February, Mr Siman-Tov said to a custody officer “I told them I was going to kill myself” and swallowed a handful of tablets, the report found.
At hospital he told a doctor he had taken four days’ supply of ibuprofen and codeine, with the intention of killing himself.
The report found he did not complain of any symptoms usually associated with an overdose, but had a fast pulse rate and significantly abnormal heart rhythm.
He was tested for levels of paracetamol and aspirin but not the two drugs he said he had taken, the ombudsman said.
Mr Siman-Tov was later discharged but the ombudsman criticised the lack of “proper discharge information… which led to a poor handover of information to nurses”.
He was sick three times between leaving the hospital and returning to his room and was found unresponsive by staff at about 03:15 BST.
A post-mortem concluded he “probably died from morphine and codeine toxicity”.
The ombudsman also “found several weaknesses” in suicide and self-harm procedures, “most notably the lack of input from healthcare staff despite Mr Siman-Tov having obvious mental health issues”.
It made five recommendations including ensuring all detainees discharged from hospital have a care management plan in place, and that all nursing staff properly check detainees required to take medication under supervision.
A Home Office spokesman said:”The welfare of all those in our care is of the utmost importance and we are grateful to the Prisons and Probation Ombudsman for his report.
“Recommendations made following any investigation into a death are treated seriously and action is taken to ensure they are implemented swiftly.”