The league tables based on tests sat in 2008 remain unpublished
There is a "significant" risk that pupils in England could face a repeat of the Sats test fiasco of the summer 2008, the exams watchdog has warned.
Edexcel has now been appointed to run the tests in 2009, after results were delayed following a series of blunders by the then operator ETS Europe.
But the exams watchdog, QCA, says the rush to appoint a new firm means there is no time to test the marking systems.
Education Secretary Ed Balls said he accepted the results may be late.
In 2008, hundreds of thousands of 11 and 14-year-olds were left waiting months for their marks from the exams.
An inquiry has been held into the delay, which also resulted in the postponement of league tables.
Lord Sutherland's inquiry blamed the exam watchdog - the Qualifications and Curriculum Authority (QCA) - and ETS Europe, the private contractor which ran the tests.
ETS Europe was sacked from its five-year contract with the QCA in August.
Half of the Sats themselves - those taken by 14-year-olds - have been scrapped in a subsequent shake-up of testing.
The former contractor, Edexcel, has been chosen to run next year's remaining tests for 11-year-olds, and it has agreed to deliver 99.9% of test results to candidates by 7 July 2009.
But the QCA chief executive Ken Boston - writing to Education Secretary Ed Balls before his suspension in mid-December - warned "whilst the supplier is committed to achieving it (the deadline), there is currently no guarantee that events will not cause them to miss it."
He stated the speed with which Edexcel had to be appointed meant the fine details of the contract will have to be worked out after it is agreed.
The risk of missing the deadline would, he said, "be at its greatest in the crucial and short period between the beginning of marking and the completion of results data".
In a reply, Mr Balls said he understood Mr Boston's concerns.
"I understand your concerns that the overall risks to delivery remain high and that there is a cumulative risk the contractual target of 99.9% of results available, on or by 7, July may not be met," he wrote.
He urged the QCA to arrange for "effective contingencies" to be put in place, "thereby reducing the likelihood and impact of serious risks to delivery".
But Mr Boston's reply to that letter re-iterates his warning: "We will do all we can to mitigate risk, but it is important that I re-emphasise to you that the risks in this process are high.
"In the restricted time available to us it will not be possible to develop mitigations to the level that either you or I would wish.
"In particular during the crucial short period between the tests being sat, and the results being published on or by 7 July, the only meaningful contingency available is a delay in publishing the result."
The written exchanges between Mr Balls and Mr Boston feature on the QCA's website.
Mr Balls has asked the QCA to give him a progress report in January.
The managing director of Edexcel, Jerry Jarvis, said: "We understand that efficient results and quality of marking are very important to students, parents and teachers and have reassembled the team which successfully delivered the contract in 2005, 2006 and 2007."