The agency After Adoption provided support to around 8,000 people each year for almost 30 years across England and Wales, working with more than 70 local authorities.
A notice of the closure released by the Consortium of Voluntary Adoption Agencies (CVAA) said the organisation had been "affected by a disruptive reform programme implemented in a time of year-on-year austerity, leading to short-term contracts, poor commissioning and insecurity for independent providers".
The agency's website announced the closure last week, with services in some areas transferred to alternative agencies, and advising that paying service users should seek compensation through the administrators.
Ofsted rated the agency "outstanding" in 2015 and then "good" in March 2018.
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The Department for Education has been leading a programme of adoption reforms since 2014, which included the launch in 2017 of Regional Adoption Agencies (RAAs) that brought together adoption placement and support services of local authorities and voluntary sector agencies across entire regions.
CVAA's statement said the RAAs had impacted negatively on After Adoption.
"The shifts in commissioning practices being experienced as part of the formation of RAAs has led to a lack of system business continuity with transitional arrangements resulting in diminished duration of contracts and pricing frozen at an outdated cost base," it said.
The government's reforms also included the creation of an Adoption Support Fund (ASF) that funded therapeutic support for adoptive parents and their children.
But in September 2018 the government dropped a five-working-day target for applications to be completed, which followed a cap imposed in 2016 on how much a child could claim.
CVAA said the delays in ASF applications and increasing late payments had "compounded" the financial pressures faced by After Adoption.
"The architects of the adoption reform programme have failed to manage the entirely predictable impact of their market disruption on independent providers, with the disastrous results we now see for After Adoption affecting many children and families," the notice said.
News of the closure comes despite the charity being awarded a three-year National Lottery Community Fund from January this year, worth more than £182,000.
Information about the charity's finances published on the Charity Commission website show steady income and spending of between £4m and £5m between 2014 and 2017.
However, the charity failed to report information on its finances for 2018.
Dear Friends— After Adoption (@afteradoption) April 4, 2019
We are sorry to have to confirm that After Adoption has now been closed
Please follow link below to see written version of statement contained within the attached imagehttps://t.co/rqtFniyIGK pic.twitter.com/fJqkRijvuv
The Department for Education highlighted its spend of more than £100million through the ASF, adding that it was are "looking at better ways in which digital technology can support fostering and adoption".
It continued: "We worked closely with After Adoption and will continue to work with others in the sector to make sure no families are left unsupported during this transition."
The DfE also thanked the charity Barnardo's for "stepping up to take on two existing DfE projects which were previously led by After Adoption".