Nick Clegg has warned headteachers that Ofsted will "forensically" analyse how well students in receipt of the pupil premium perform over the coming years, with reports being made directly to him.
The deputy prime minister issued the caution this week in a speech that spelled out his plans for the policy, which hands additional money to schools based on the number of pupils on roll who are entitled to free school meals.
The comments were greeted with concern by heads' and teachers' leaders, who believe the government is showing an "over-reliance" on the schools' watchdog when it comes to accountability.
The Liberal Democrat leader, speaking from the New North Academy in North London, said schools would be free to spend the pupil premium as they saw fit, but added that Ofsted would be reporting back to his office on how well the money was being spent.
"Schools need to know that, in assessing their performance, Ofsted will be looking forensically at how well their pupil premium pupils do," Mr Clegg said. "Inspectors are already being instructed to look closely at how schools are spending the money and to what effect, with plans to publish a survey early next year.
"And, because Ofsted understands the priority I attach to this issue, it will be providing me with regular reports detailing the progress schools are making in closing the attainment gap."
The premium is expected to double in size by the end of this Parliament, growing from #163;1.25 billion to #163;2.5 billion by 2015.
Mr Clegg's words of warning came in a speech that was otherwise upbeat about the teaching profession. His comments were in stark contrast to those of Sir Michael Wilshaw, Ofsted's chief inspector, who last week said that heads and their staff should stop complaining about how stressful their jobs were.
The Association of School and College Leaders (ASCL) welcomed Mr Clegg's tone, but said it had concerns over the role of the schools' watchdog when it came to the pupil premium.
"There is a worry about the over-reliance on Ofsted from central government to report what is going on in schools," said ASCL general secretary Brian Lightman. "It would be better if Ofsted could identify best practice and help share it, not promote the very unhelpful messages it has been doing in recent weeks and months."
Russell Hobby, general secretary of the NAHT heads' union, said it was disappointing that Mr Clegg had made the "threat" about Ofsted, despite saying he wants to "boost the morale of teachers".
Mr Clegg also announced plans, in partnership with TES, for a pupil premium award that will recognise the 50 schools that have done the most to narrow the attainment gap between the poorest pupils and their wealthier peers. Cash rewards of up to #163;10,000 will be on offer to the top-performing schools.