The Russian bomber which was intercepted by Royal Air Force (RAF) fighters on 28 January is thought to have been carrying at least one nuclear weapon, a revelation which could further escalate tensions between Russia and the West.
RAF Typhoon fighter jets were scrambled last week after two Russian aircraft - Tu-95 "Bear" bombers - were spotted cruising above the English Channel.
"We downloaded conversations from the crew of one plane who used a special word which meant the would-be attack was a training exercise," a senior source with the RAF told the Express.
"They know that we can pick up their transmissions and it would only be of concern if the often used release weapon order was changed. We also knew from another source that one of the aircraft was carrying a nuclear weapon long before it came anywhere near UK airspace."
The nuclear missile was not armed and would have required a direct order from Russian President Vladimir Putin to activate it.
Both Prime Minister David Cameron and Defence Secretary Michael Fallon were briefed over the situation as soon as the Russian aircraft were to be intercepted.
Russian ambassador to the UK Alexander Yakovenko earlier said those planes were not on "threatening" or "disruptive" missions in the UK airspace.
"This flight [as all other routine flights of the Russian military aircraft] was carried out in strict compliance with international legal norms including international flight rules and regulations, without violation of other countries' airspace, therefore it cannot be regarded as threatening, destabilising or disruptive," the Russian embassy said in a statement.