England's best academy chains are raising standards for their poorest pupils and outperforming other state schools, but the weakest are lagging far behind, research suggests.
It concludes that there is "enormous variation" between chains, and the results and improvement of their disadvantaged pupils.
The research, commissioned by the Sutton Trust, analysed school performance figures to look at what disadvantaged pupils attending schools in an academy chain achieve based on five different measures.
These are: proportion of pupils getting at least five C grades at GCSE - including English and maths; expected progress in English; expected progress in maths; the proportion achieving the English Baccalaureate and average GCSE points score.
In total, 31 chains were included, all with three or more academies.
The findings show that across the five measures, between 2011 and 2013, disadvantaged students at academies in 18 of the 31 chains improved faster than the national average.
Overall, in this two year period, the proportion of poorer teenagers nationally - at all mainstream schools - that scored at least five C grades, including the basics, rose by 4.2 percentage points.
The report shows that in 16 of the chains examined poorer pupils' improvement on this measure alone was greater than this national average.
It goes on to say that five chains improved significantly more than this 4.2 percentage point national average. They are - Barnfield Education Partership, the City of London Corporation, the David Ross Education Trust, the Diocese of Salisbury and Leigh Academies Trust.
The study also found that based on last year's GCSE results, poorer pupils educated by nine academy chains achieved higher grades than the average for all mainstream schools.
Researchers went on to look at the proportion, and improvement in disadvantaged pupils gaining the English Baccalaureate (Ebacc).
The Ebacc is a performance measure that recognises pupils who score at least a C at GCSE in English, maths, science, a foreign language and history or geography.
In total, 8.3% of poorer teenagers in the 31 chains analysed achieved the Ebacc, compared to 10.2% of pupils in all mainstream schools.
On this measure, seven chains saw a faster improvement in results between 2011 and 2013 than the national average.
One of the authors, Professor Becky Francis, of King's College London, said: "Some chains are securing excellent results for their disadvantaged pupils across a whole range of measures, showing what can be achieved her by experienced chains with a planned approach to growth.
"Their work should be recognised and applauded. But others are doing badly on important measures, and risk becoming part of the problem rather than the solution for their disadvantaged pupils. The government needs to increase transparency and scrutiny of academy chains."
The report comes amid calls by Ofsted chief Sir Michael Wilshaw for the schools watchdog to be given explicit powers to inspect academy chains, in the same way that they can inspect local council children's services.
In recent months, Ofsted has attempted to overcome the issue by publishing findings of inspections at a number of academies run by two different academy chains, where there had been concerns about performance.
Just last week, the inspectorate warned that an''overwhelming'' proportion of pupils attending some of the Kemnal Academies Trust (TKAT) schools are not getting a decent education.
In a highly critical letter to the trust, the watchdog said that while the trust was getting better at supporting its schools, in many cases this had come too late.
TKAT said it was ''committed to educational excellence'' and is working with its schools to make changes.
Inspectors visited six TKAT primary academies over a two-week period amid wider concerns that standards in the Trust's schools were not up to scratch.
And earlier this year, Ofsted sent a similar letter to the E-ACT Trust - one of England's biggest academy chains, warning that it had failed to take effective action to improve standards in many of its schools.
E-ACT said it has been making changes for over a year and introducing reforms under new leadership.
A Department for Education spokesman said: "We welcome the Sutton Trust's report which underlines the hugely valuable work being done by academy chains across the country to improve the lives of thousands of disadvantaged pupils.
"The excellence of established chains such as Ark and Harris, along with outstanding newer trusts such as the Gorse Academies Trust, Reach2 and Outwood Grange Academies Trust have turned around schools that had failed children for years when in local authority control. They have been able to give their heads and teachers the freedom they need to instigate real improvement."