Exam regulators will not be able to guarantee the quality of marking of this year's key stage 2 Sats because of the rush to find a new company to run the tests following last year's shambles.
The Qualifications and Curriculum Authority has told the Government that the system provided by the new contractor, Edexcel, will limit its ability to respond to any marker mistakes.
It has also warned ministers that there will no opportunity to test the new Sats system fully and that it could be vulnerable to a campaign of industrial action aimed at undermining the tests.
The admissions came to light in a letter to Ed Balls, the Schools Secretary, written in November outlining the emergency arrangements to bring in Edexcel after the American firm ETS had its five-year contract prematurely terminated.
Sir Ken Boston, the QCA chief executive, was suspended on full pay last month when the report of an inquiry into the problems was published. He writes: "Due to the use by Edexcel of a paper-based management and information system, there will be delays and lags in providing information on marker performance. This will limit the ability to respond to any shortfalls in marker performance."
Last month's Sutherland inquiry into the ETS debacle exonerated markers, but recommended that they and teachers should be given more say in marking from 2009.
The troubles last summer delayed results for 1.2 million pupils and left schools and markers frustrated by disorganised training, multiple computer glitches, missing scripts and a failing helpline.
Sir Ken told ministers that if there were any problems, particularly during 2009's "crucial, short" marking period, the only option would be to delay the Sats results for a second consecutive year.
His letter warns in three separate places that the July 7 delivery deadline for marked papers could be missed because of the "very significant nature of all the risks involved".
The QCA also says it does not have all the powers it wanted to supervise Edexcel in the Pounds 25m contract and will only have a limited ability to step in if things go wrong.
The high risks stem from only having nine weeks to find a new contractor and carry out a procurement process that usually takes between 9 and 12 months.
Half of the initial six bidders withdrew because they were either concerned about meeting the requirements in such a short time or felt the level of risk they faced was unacceptable.
"We will not be in the same strong position as with the ETS contract," Sir Ken warns. "The lack of other solid bidders has worked against us."
Mr Balls wrote back on December 8 urging the QCA to provide reassurance that Edexcel's system would work.
But Sir Ken told him: "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."