A think-tank has renewed calls for an end to "birthday discrimination" in entitlement to free nursery education.

Reform Scotland said the current system for government-funded pre-school provision was "unacceptable and unfair" as some children are entitled to less early-years education than others.

The organisation highlighted that as the legal entitlement starts the term after a child turns three, a child born in August will receive a full two years before starting school while a child born in September will receive only 18 months.

Reform Scotland said the cost to parents using private nurseries could be as much as £2,800 and argued the scale of the "unfairness" would increase as the government moves to double the current entitlement.

In response to a Scottish Government consultation on the expansion, the think-tank called for provision to begin in the August two years before a child is due to start school, irrespective of their age at the time.

Research director Alison Payne said: "The Scottish Government has set a goal of closing the attainment gap.

"However, many children start school already at a disadvantage and playing catch up simply because they have not been given the same access to pre-school education.

"While many factors contribute towards the attainment gap, it is unacceptable that one of those is as a result of the Scottish Government's birthday discrimination which could be so easily resolved.

"The Government's commitment to quality childcare and improving educational outcomes is clearly expressed, but significantly undermined by this unfairness based on nothing more significant than the month in which a child was born."

The Scottish Conservatives also called on the government to take urgent action to address the "anomaly".

The party's education spokeswoman Liz Smith said: "For several years, we have urged the Scottish Government to address this discrimination but our plea has been steadfastly refused at every turn.

"If the SNP doesn't want to listen to us, then it should at least pay attention to this warning from Reform Scotland.

"It is plainly wrong that a child loses out on hundreds of hours of nursery education purely because he or she was born in the 'wrong' month."

A Scottish Government spokesman said: "Our commitment to increase free early learning and childcare to 1,140 hours by 2020 is this government's most transformative infrastructure project.

"Work has already started to plan for this expansion and we continue to work with our partners as we take forward our ambitious pledge.

"The starting dates for three-year-olds are set out in legislation, however local authorities can and do start children earlier where they have capacity to do so, and this is for them to fund and manage.

"Children who start their entitlement in April can have an extra year of funded early learning and childcare if parents choose and children who start their entitlement in January are eligible for an extra year at the local authority's discretion."