The former British Ambassador to Moscow has warned that if Russia has invaded Ukraine then it would be difficult for Britain to avoid going to war.
Sir Tony Brenton, who served as British Ambassador from 2004 to 2008, said that war could be an option 'if we do conclude the [Budapest] Memorandum is legally binding.'
The Budapest Memorandum was signed in 1991 by Bill Clinton, John Major, Boris Yeltsin and Leonid Kuchma – the then-rulers of the USA, UK, Russia and Ukraine.
It promises to protect Ukraine's borders, in return for Ukraine giving up its nuclear weapons.
Today Kiev has demanded the agreement is activated after insisting their borders had been violated.
In response Mr Brenton said in a BBC radio interview: 'If indeed this is a Russian invasion of Crimea and if we do conclude the [Budapest] Memorandum is legally binding then it's very difficult to avoid the conclusion that we're going to go to war with Russia'.
Ukraine accused Russia of a 'military invasion and occupation', saying Russian troops have taken up positions around a coast guard base and two airports on its strategic Crimea peninsula.
Russia kept silent on the accusations, as the crisis deepened between two of Europe's largest countries.
The Budapest Memorandum was signed in 1991 by Bill Clinton, John Major, Boris Yeltsin and Leonid Kuchma - the then-rulers of the USA, UK, Russia and Ukraine. It promises to protect Ukraine's borders, in return for Ukraine giving up its nuclear weapons
Any Russian military incursion in Crimea would dramatically raise the stakes in Ukraine's conflict, which saw pro-Russian President Viktor Yanukovych flee last weekend after three months of anti-government protests.