By the time they are five, pupils from poorer backgrounds can be up to a year behind their middle-class peers developmentally, and lack basic skills such as literacy and problem-solving, according to a study by the Joseph Rowntree Foundation (JRF).
Fresh Scottish Government figures also show a poverty fault-line has emerged, with the wealthiest 30% of households owning 81% of all property wealth.
Today's study, by the Office of National Statistics (ONS), also found almost one-third of Scottish children live in households with no material wealth, and their parents do not own their own home.
The SNP Government has blamed the lack of control over key economic and social policy levers for the growing chasm between rich and poor.
But ministers have been criticised for insisting on continuing policies including the council tax freeze, free university tuition for all Scots and EU students, free prescriptions for all and free bus travel for the over-60s as Government budgets are squeezed.
Deputy First Minister Nicola Sturgeon said the "alarming figures" in the ONS's Wealth and Assets Survey 2008-10 showed more than one-third of children were not getting a fair start in life.
She said: "Our poorest households do not have the income needed to gain the wealth and security that comes from owning property or having pension wealth. Unless action is taken soon, this cycle of deprivation will continue, with more children continuing to be born into poverty.
"Tackling and reversing this inequality requires key economic and social policy levers being in the hands of the Scottish Government."
The JRF report found that by early secondary school, 28% of children from poorer families are performing well in numeracy, compared to 56% of those from advantaged backgrounds. Children from poorer families are also more likely to leave school early and without a qualification, according to the research.
The study, carried out by academics at Strathclyde University, suggested the ministers should raise awareness and provide clear national guidance on how schools can close the gap.
Jim McCormick, Scotland adviser at the JRF, said too many poorer pupils are at risk of a lifetime of poverty.
He added: "This limits their life chances and prospects, which not only has a knock-on effect for them through unfulfilled potential, but for Scotland's economic performance.
"At all ages and stages in a child's school career, there are interventions which can be made to break the cycle of low attainment. Closing the attainment gap must be a higher priority."
Larry Flanagan, general secretary of the Educational Institute of Scotland (EIS) teaching union, said tackling poverty presented "significant challenges" for the education system, particularly at a time of cuts.
He said: "In an environment where one out of every five children in Scotland lives in poverty, it will require a co-ordinated and sustained effort from all parts of society to improve the situation for our young people."
Scottish Labour's health spokeswoman Jackie Baillie MSP said: "The SNP will blame everyone other than themselves for what is happening in Scotland. The Coalition Government has much to answer for, but it isn't an excuse for inaction at Holyrood.
"After almost eight years in government, the SNP really have to stand on their record. Under Labour, child poverty in Scotland fell dramatically. Under the SNP that progress has stalled.
"The SNP have stood by while the educational attainment gap between our poorest children and our wealthiest remains stubbornly high.
"Health inequalities continue, without any consistent focus on reducing them.
"This isn't a government; they have been reduced to a campaign."
Scottish Conservative welfare reform spokesman Alex Johnstone added: "Of course, the Scottish Government has done nothing to address this in seven years of power. It's easy for the SNP to lay everything at Westminster's door, but the reality is they have absolutely no realistic answers on how to address this statistic themselves."
Neil Mathers, the head of Save the Children for Scotland, said: "The attainment gap between children living in poverty and their peers is unacceptable and the consequences can be lifelong.
"It often means opportunity is out of reach and can lock children into a cycle of poverty.
"Save the Children has long called for this issue to be addressed and for better understanding of the causes of the attainment gap in Scotland to be made a priority."