Former British defence minister and Nato secretary general George Robertson.

A former secretary general of Nato has warned that Scottish independence would be “cataclysmic” because breaking up Britain would rob the West of a major anchor of stability and solidarity at a difficult time.

George Robertson, a former British defence minister and Nato head from 1999 to 2003, said a vote for independence at a September 18 referendum would erode Britain’s role in the West and undermine the global balance of power.

Robertson, a Scot, said Scotland ending its 307-year tie with England would have consequences across Europe, triggering a possible fragmentation as breakaway movements in Catalonia and the Basque country in Spain and in Belgium followed suit.

“The loudest cheers for the break-up of Britain would be from our adversaries and from our enemies,” Robertson said in a speech delivered at the Washington-based US think tank, the Brookings Institution, late on Monday. An audio recording of the speech was made available on the think tank’s website. “For the second military power in the West to shatter this year would be cataclysmic in geopolitical terms ... Nobody should underestimate the effect all of that would have on existing global balances and the forces of darkness would simply love it.”

He said splitting up the United Kingdom, Washington’s staunchest ally on the global stage, would lead to a torrid, complex divorce that would take years and deprive the West of a serious partner at a time “when solidity and cool nerves will be vital” amid renewed territorial intimidation.

He warned the Scottish National Party’s (SNP) promise to get rid of Britain’s Trident nuc-lear submarine fleet from western Scotland would also have wider implications.

“It is one thing to unilaterally disarm yourself but to unilaterally disarm your neighbour is playing with fire,” Robertson said. “The ripple effects will go much wider than our own shores.”

Arguing this was not a purely domestic matter, Robertson urged the US government and other UK allies to make their views public as Scottish independence would affect them too by undermining stability in the region.

Robertson’s comments came as Scottish First Minister Alex Salmond was in New York on a trade mission.