SNP plans to spend up to £1.5 billion a year on foreign aid after independence have been dismissed as total fantasy by the UK's International Development Minister, writes Tom Gordon.

Tory Alan Duncan also warned 600 jobs would be at risk in East Kilbride in the event of a Yes vote, and dismissed the idea of working with an independent Scotland's foreign aid department, snapping: "Why should we?"

Duncan's comments, in a briefing to the Scottish press, are a direct challenge to SNP External Affairs Minister Humza Yousaf.

Yousaf wants an independent Scotland under the SNP to spend at least the UN target of 0.7% of national income on foreign aid, or £1bn a year but ideally 1%, or £1.5bn a year.

The SNP Government currently spends £9 million a year in Malawi, Zambia, Tanzania, Rwanda and south Asia: spending £1.5bn would represent a 166-fold increase.

Duncan said: "Not credible. If you believe that, you'd believe anything. This adds to a picture of reduced credibility - along with the problem with currency, membership of the EU, and the ability to attract investment."

Duncan said the aid impact of an independent Scotland would be less than the SNP imagined, and far less than his Department for International Development (DfID) achieved.

"They [Scotland] are just not going to be able to deliver the development effectiveness we think is so important for helping some of the poorest and most vulnerable people in the world. If you split us [DfID], you're diminishing both sides."

He also said a Yes vote would force DfID to reconsider its "whole operating model", including Abercrombie House in East Kilbride, home to a large number of DfID's 1300 UK staff. "I don't want 400 or 500 people in Abercrombie House to have their livelihoods at risk," Duncan added. "We'd rather stick together."

Duncan seemed hostile to working with the government of an independent Scotland. Asked if there would be a partnership between the remainder UK and an independent Scotland's international development agency, he replied: "I can't say it's an easy Yes. Why should we? It's value for money, isn't it? My taxpayers' money."

A Scottish Government spokeswoman said: "We are committed to preserving continuity of employment for all staff based in Scotland, and to work with the unions and staff involved. The real risk to jobs at DfID comes from the UK Government's failure to protect the aid budget in law. Without this there is no guarantee the UK aid budget will not be cut in future."