Rhodri Morgan warned that Scottish independence would leave the Welsh and Northern Irish even more outnumbered by the English.
The disparity would mean Wales might not have a "sustainable" position in the UK that remained, he writes in an article for Prospect magazine.
But it is perhaps his colourful depiction of the disparity within the UK that is the most eye-catching. The Labour politician admits that he once upset Scotland's First Minister Alex Salmond by "referring to the UK as comprising an 'Elephant and Three Fleas'".
The SNP leader accused him of a "form of Celtic self-loathing," he says.
"I told him that you had to 'clock' the enormous disparity between England and the rest.
"And I thought then, as I still think, that we fleas ought to stick together."
A Yes Scotland spokesman said it was "commendable" that Mr Morgan was standing up for Welsh interests.
But he added: "However, his proposition that Scottish independence will weaken Welsh and Northern Irish voices in the UK is wrong-headed.
"A Yes vote next year will complete Scotland's home rule journey, but it will also benefit the rest of the UK by helping to redress the huge economic imbalances currently in favour of London and the south-east of England.
"Wales, Northern Ireland and English regions, which currently lose out because of this over-concentration in one part of the UK, can therefore share in the benefits of a Yes vote for Scotland next year."
There is increasing concern in Wales over the potential impact of a Yes vote in next year's Scottish independence referendum.
Last month Welsh First Minister Carwyn Jones called for a veto on any currency union between the UK and an independent Scotland. He said: "If one part of the currency union decides to leave, then that is a matter for them."