THE SNP's plan for a £2.5 billion annual security budget in an independent Scotland has been branded "back of the envelope stuff" and would put Scots at risk, a defence seminar has been told.

Sir Malcolm Rifkind, the former Conservative foreign, defence and Scottish secretary, speaking at the Royal United Services Institute in London, suggested the SNP's security prospectus contained fundamental flaws. He pointed out that the suggested 15,000 strength for a Scottish defence force would result in combat forces of no more than 5000 given two-thirds of personnel would be engaged in such things as engineering, logistics, medical care and intelligence.

"So the numbers have to take that into account and, if the 15,000 do take that into account, then the combat forces are no more than 5000," said Sir Malcolm.

"Is that something we should enthusiastically look forward to as being something that would enhance Scotland's position in the world? This is back of the envelope stuff."

Sir Menzies Campbell, the former Liberal Democrat leader, also complained of a lack of detail, pointing to engineers, artillery, medical, training and procurement.

Lord Browne, the former Labour defence secretary, also took issue with the SNP's post-independence defence budget of £2.5bn, arguing that an "extraordinarily expensive series" of systems would have to be established such as a new command and control centre, a Ministry of Defence and intelligence structure.

He said this might be able to be done by using up a substantial part of the £2.5bn "but it won't leave you very much".

The Labour peer said the UK, because of a shared approach, enjoyed a world-class level of security, warning: "We should not give any of that away, even for a comparative short period of time easily, because it's an extremely risky thing to do."

In response, Angus Robertson, the SNP's defence spokesman, drew a comparison with Denmark, which was of a similar size to Scotland, and had a 2012 military budget of £2.6bn. He accepted that 15,000 was the "all arms figure".

He stressed independence would allow Scotland to decide which military operations to become engaged in, referring to Iraq. He said: "A nation should be able to decide in essence does it send its young men and women to war or not; key question – Iraq? No thank you."