Gordon Brown has suggested it is unfair for students from England paying tuition fees to study in Scotland while Scots and EU nationals get the same education free.
"There are hard truths we in Scotland have got to face up to," he said. "I don't believe it's right, personally, that we believe in social justice, and yet we have tuition fees free to Scotland to Scottish students, free to anyone from the west of the European Union, and we charge someone who comes from England.
"I think that will increasingly become a difficulty, because tuition fees have gone up a lot," he said.
"I'm not saying there's an easy solution to it, but there should be a negotiation about it. It seems to me not in tune with the principles I support, of social justice," he added.
Tuition fees were first introduced across the UK by New Labour in 1998 when Gordon Brown was chancellor, following recommendations in the Dearing report, which had been commissioned by the previous government. Following devolution Scotland moved towards an endowment system in 2000 before abolishing the fees altogether after the SNP formed a minority government in 2007.
Brown made the comments during a lecture to a public meeting of the Scottish Parliament in Edinburgh on Friday evening, during which he also warned that Scottish independence would create a "race to the bottom" in public services, with England and Scotland trying to undercut each other's tax and spending policies.
The former Labour prime minister warned that the two countries would end up cutting corporation tax to seem more attractive to businesses, and that public spending would suffer as a result.
"The argument goes that we could have separate rates of corporation tax. It sounds quite an attractive argument, doesn't it? Companies could choose which country to go to, but they're paying different rates of corporation tax and the cheaper rate would be attractive to one set of companies, which would move out of one country and into another.
"It's what Ireland did famously and which was hailed for a time as a success. But the Irish corporation tax cut was funded by money from the European Community," he said.
"But if you have less money in the country with the lower corporation tax rate, you then have less public services as a result."
"It may seem superficially attractive but I would say it is against the history of social justice," he concluded.
Brown spoke for an hour at Holyrood on themes of social justice, suggesting that the Scots were "internationalists" who had contributed to social justice across the UK, pointing out that many union leaders were Scottish. He said that the pooling together of resources was central to the Scottish notion of social justice, and that this lent itself to the UK staying together.
"I think there is a danger that the referendum debate becomes about process and procedure, and not about principles," he said.
"There is a danger that we get into a Westminster or Holyrood mentality of talking about the minutiae of detail about running the referendum
“The more fundamental issue is what is in the people of Scotland's interest?" he concluded.
The Scottish independence referendum is almost certain to take place in the autumn of 2014, after David Cameron in effect abandoned his campaign for it to be held sooner. London and Edinburgh remain divided over the structure of the referendum and on whether it should consist of a straight yes/no question to independence. The SNP government is pushing for a third option of greater devolution, often branded Devomax, to be put on the ballot paper.
But Alex Salmond's plans suffered a major setback earlier this week when legal experts concluded that only London had the power to call a referendum and set the question. A separate report by MPs suggested having three possible answers on the ballot paper would be confusing and risked jeopardising the validity of the outcome of the referendum.
Responding to Brown's comments on tuition fees the SNP said in a statement: "Mr Brown’s attack on the Scottish Government’s policy of maintaining free education for Scottish students was simply bizarre. Thanks to having effective independence over universities policy in the Scottish Parliament, the SNP has delivered a policy based on ability to learn, not ability to pay – and our student numbers are going up in Scotland, while they are plunging in England due to Tory cuts.
"Yet it sounds a though Mr Brown wants our universities to be controlled by Westminster, instead of being a Holyrood responsibility."
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