A CLOUD of radioactive particles is floating across Europe — and no one knows where it came from.

First detected in mid-January, spikes in the level of a radioactive isotope called Iodine-131, have been recorded all the way from Norway to Spain.

Iodine-131 (I-131) is produced by nuclear fission in nuclear explosions, power reactors, and industrial and medical isotope facilities.

It has led some to ponder whether Russia has begun secretly conducting nuclear weapons tests.

One of the Soviet Union’s largest atomic bomb testing grounds was on the island of Novaya Zemlya, deep in the Arctic Circle, off the coast of Norway.

While nuclear monitoring organisations believe this theory is unlikely, what has caused the radioactive plume remains a mystery.

The US military has now dispatched a nuclear ‘sniffer plane’ to the UK, that can detect and analyse radiation.

Although the US military deny it, some believe its mission is to track down the source of the spikes.

The alarm was first raised in January by the IRSN, the French Radioprotection and Nuclear Safety Institute.

The public body said the cloud of I-131 had first been detected in northern Norway in the second week of January. Norway’s remote north, above the Arctic Circle, borders Russia.

The isotope was subsequently detected in Finland, Poland, the Czech Republic, Germany, France and finally in Spain, as the wind carried it from the north of Europe to the continent’s south west.

At high enough levels I-131 can cause deadly thyroid cancers. However, it also used in radiation treatments to combat cancers.

In a statement, the IRSN stated “Iodine-131 is a radionuclide with a short half-life. The detection of this radionuclide is proof of a rather recent release.” So brief is I-131’s eight day half-life, the origin of the release must have occurred in early January.

However, the organisation stressed there was “no health concerns” due to the release.

With the source of the release still unknown, the US has sent one of its two Boeing WC-135 nuclear ‘sniffer planes’ to the UK.

Flying into an air base in eastern England last week it has now reportedly been sent to northern Norway.

Named the Constant Phoenix, the converted jet is specially designed to test the air for signs of radioactive leaks and nuclear explosions.

The aircraft has been spotted close to North Korea following the country’s nuclear missile tests as well as over Japan in the days after the Fukushima power station meltdown.

The US military has confirmed the Constant Phoenix’s presence in Europe but played down its significance.

An US Air Force spokesman said the aircraft was “on a pre-planned rotational deployment scheduled far in advance,” adding, “anything contrary is completely baseless,” reported the independent.

The converted military jet is nicknamed the ‘sniffer’ or ‘weather bird’ by its crews who are kept to a minimum when radiation is suspected in the atmosphere to avoid danger to personnel on board the aircraft.

Two scoops on the side of the plane suck in gases which are then trapped on filters. Crew on the aircraft then analyse the particles to check for radioactivity.

A positive identification of I-131 by the plane is one of the telltale signs of a nuclear explosion.

But other key indicators, such as non-natural seismic activity, have not been reported casting doubt on whether an explosion is the leak’s cause.

In addition, no other radioactive material has been picked up

The Comprehensive Test Ban Treaty Organisation, which polices various Governments’ commitments to not test nuclear weapons, said that while I-131 had been recorded it was within “local historical levels”.

“If a nuclear test were to take place that releases I-131 it would also be expected to release many other radioactive isotopes. No other nuclear fission isotopes have been measured at elevated levels in conjunction with I-131 in Europe so far,” the organisation said in a statement this week.

One explanation is that the isotope may have leaked from a pharmaceutical facility where I-131 is produced for medical reasons.

However, no pharmaceutical lab has come forward to claim the I-131 as their own.

Another possibility is that the plume may have escaped from a Russian nuclear submarine sailing through the Arctic.

The mystery comes at a time of deep wariness between Russia and the US following the former’s annexation of the Crimea and the backing of armed rebels in Eastern Ukraine.

Just last week, it was revealed Russian jets buzzed a US destroyer off the coast of Romania which the Navy slammed as “unsafe and unprofessional”.