A "growing" number of schools are using their funding for disadvantaged pupils to offset budget cuts elsewhere, new polling shows.
In a survey, 6% of teachers reported this as the main priority for their school's pupil premium spending - an increase from 2% in 2015.
The research, published by the Sutton Trust and the Education Endowment Foundation, involved 1,607 teachers.
One in five of respondents did not know what the main priority for their pupil premium funding was.
But 3% of senior leaders and 7% of classroom teachers - a total of 6% - stated that it was for "offsetting budget cuts elsewhere".
The pupil premium, £1,320 per primary pupil in the current financial year and £935 for secondary pupils, was introduced in April 2011.
It is paid for pupils who have been eligible for free school meals over the previous six years or who have been in care.
Schools also receive £1,900 for pupils who have been in care but are now adopted or have left care under certain guardianship orders.
The pupil premium currently costs £2.5bn each year, more than 6% of the schools budget.
Sir Peter Lampl, chairman of the Sutton Trust and of the Education Endowment Foundation, said: "It is worrying that a growing number of schools feel they have to use funding for disadvantaged pupils to offset budget cuts.
"The pupil premium is a key lever for raising the attainment of disadvantaged pupils and it's vital that it continues to be focused on their education."
The survey, conducted by the National Foundation for Education Research, showed one in five teachers did not know what the main priority for their pupil premium funding was.
In total, 28% of teachers said their priority was on early intervention schemes, while 13% identified theirs as more one-to-one tuition, and 10% said teaching assistants.
Virtually all of the senior leaders surveyed said the pupil premium had allowed them to target resources at raising the attainment of their poorest pupils.
The polling also highlighted the increasing use of research evidence in the teaching profession.
Of the secondary senior leaders, 63% said they considered research evidence on the impact of different approaches and programmes when deciding which to adopt to improve pupil learning.
Six out of 10 said they used the Sutton Trust-EEF Teaching and Learning Toolkit, a summary of educational research developed with a team of academics at Durham University.
Sir Kevan Collins, chief executive of the Education Endowment Foundation, said: "It's great to see a majority of school leaders are now using our Teaching and Learning Toolkit to decide which programmes and approaches to adopt.
"Better use of research can help schools spend their resources in the most efficient ways and make a real difference to outcomes for disadvantaged pupils."
A spokeswoman for the Department for Education said: "The pupil premium - worth £2.5bn this year - is helping the most disadvantaged pupils and closing the attainment gap at both primary and secondary level.
"We trust schools to use the money to best meet the needs of their pupils and are pleased this report shows the vast majority of school leaders feel they can target it to support their poorest pupils.
"As we set out in the White Paper, we are taking further action to improve the effectiveness of pupil premium spending and schools continue to be held to account for using the pupil premium effectively through performance tables and Ofsted inspection.
"We have protected the core schools budget in real terms, so that as pupil numbers increase so will the amount of money in our schools.
"At the same time we are consulting on a national formula to make funding fairer, and address the historic unfairness in the system so funding is matched to need."