Care homes in England were planning their response to the coronavirus pandemic "with their hands tied" because data about outbreaks in the sector were not published until the end of April, an industry body has said.
Public Health England had counted more than 4,500 Covid-19 outbreaks in care homes before it issued its findings, figures reveal.
The National Care Forum voiced concerns at the data not being shared but PHE said it was used by health protection teams and to brief ministers.
Gary Lemin, whose father Roger died from Covid-19 in Cornish care home Roseland Court, told BBC Radio 4's File on 4 that a lack of data meant care homes have been "fighting a losing battle".
He added: "It makes it very difficult to understand the problem. It's almost as if their lives don't matter as much as anybody else's.
"It's a kind of an indictment of the way that the care system has been seen over this crisis."
It comes as figures from the Office for National Statistics and its counterparts in Scotland and Northern Ireland suggest more than 11,600 people have died with coronavirus in care homes across the UK since the start of the pandemic.
PHE has been collecting data on the number of both suspected and confirmed outbreaks in English care homes since 9 March.
The data shows there were more than 500 Covid-19 outbreaks in care homes in the week beginning 23 March - with this figure increasing to almost 800 the week after.
In the week beginning 13 April, there were nearly 1,000 outbreaks in English care homes.
The South East region recorded a more than five-fold increase in outbreaks within one week in early March, at a time when other regions' homes had barely registered cases.
By the time PHE published the information - on 29 April - there had been more than 4,500 outbreaks in care homes.
'Game of catch up'
Vic Rayner, executive director of the National Care Forum, which represents more than 120 not-for-profit care organisations, said: "The consequences of not having that data are huge.
"It has affected our ability to plan, prioritise, identify early outbreaks and bring in the right level of medical and health expertise.
"Having that overall picture of knowing what's going on is absolutely critical. I think it's impossible to operate effectively without that."
"We're now in a terrible game of catch up."
PHE said it "used the data on reported outbreaks in care homes, large and small, to inform directors of public health, directors of adult social care… and other partners through LRFs (local resilience forums)".
But the Local Government Association (LGA) confirmed it had not received the data and told the BBC that directors of public health have said it has been "difficult" to get postcode level data - and that real-time data sharing has been a problem both nationally and locally.
The LGA added: "Data on testing, deaths and better surveillance will be required as we move into contact tracing and case finding."
James Bullion, director of the Association of Directors of Social Care, said he was not aware of the PHE outbreak figures.
He added: "We were all, as directors, wanting greater distribution, greater understanding and transparency of the data around outbreaks and indeed, around incidents."
PHE said it only published the data in late April after introducing detail about the number of outbreaks at local authority level in order to help support government.
A PHE spokesman added: "PHE's health protection teams play a vital role locally in responding to any outbreak in care homes, providing tailored infection control advice to allow staff to protect themselves and their residents."
The Department for Health and Social Care said: "The government's daily figure now includes deaths that have occurred in England in all settings where there has been a positive Covid-19 test, including hospitals, care homes and the wider community."
Coronavirus: The care homes catastrophe was on BBC Radio 4 on Tuesday 19 May at 20:00 BST and is available to listen now on BBC Sounds.