Reaching the expected level in maths and English at the end of primary school is no guarantee a pupil will go on to achieve well, an MP warns.
Liberal Democrat Simon Wright says less than half of pupils in England who just manage a Level 4 in national curriculum tests (Sats) will get five good GCSEs.
Primary school achievement must be raised to help children get decent exam results at the age of 16, he said.
Ministers are planning to introduce a new higher primary standard.
This will make the tests tougher to ensure pupils are ready for secondary school.
Currently, pupils take Sats tests in English and maths at the end of Key Stage 2, in their final year of primary school, with a Level 4 being the standard pupils are expected to reach.
Speaking at a Westminster Education Forum in central London Mr Wright, who is parliamentary private secretary to Schools Minister David Laws, said evidence suggested that Level 4C (the bottom third of the Level 4 mark range) "has turned out not to represent the milestone on the path to educational success that we would expect".
"The fact is that less than half of all children who achieve 4C at aged 11 go on to gain five good GCSEs," he said.
"Achieving that score is therefore not a guarantee of later success or even a strong indicator of later success. The fact is that a child averaging a 4C in their Key Stage 2 assessment has only a 47% chance of getting five A* to Cs including English and maths.
"That means that the majority of young people who just clear the current primary school mark will fail to achieve the current, relatively modest, performance benchmark at secondary school."
Children who score a 4A or 4B - the top two thirds of the mark range - have a 72% chance of getting at least five A* to C grades including English and maths at GCSE.
"Of course, we also want to see more children jump over the existing bar, but we must also make sure that children leave primary school ready for success in secondary school and the bar must be set at an appropriate level for that," he said.
"So the new primary standard will be higher. The pass at 11 must mean that the child is on track to be successful at 16; that will be the standard that is at least equal to Level 4B at present."
In March, Mr Laws announced that the government planned to raise the floor target on achievement in primary schools.
From next year, primaries in England must make sure that at least 65% of their Year 6 pupils score a Level 4 in English and maths, and meet national progress measures.
Under the current system, schools are expected to ensure that 60% of pupils meet this benchmark.