Image caption Gagarin's death has been surrounded by vigorous speculation down the years

New details have emerged about the air crash on 27 March 1968 that killed Yuri Gagarin - the first man in space.

Fellow cosmonaut Alexey Leonov claims an "unauthorised" plane flew too close to Gagarin's fighter jet, sending it into a spin.

Gagarin and his flight instructor Vladimir Seryogin died when their MiG-15 went down near the town of Novoselovo, about 90km from Moscow.

Secrecy surrounding the crash has led to vigorous speculation down the years.

A government investigation of the accident (which Mr Leonov was part of) concluded that the MiG tried to avoid a "foreign object" - such as geese, or a hot air balloon.

On the conclusions of this original investigation, Mr Leonov said: "That conclusion is believable to a civilian - not to a professional."

In an interview with Russia Today, the cosmonaut - who, in 1965, became the first person to walk in space - claimed he had been permitted to share a declassified report showing that a Sukhoi fighter jet flew too close to Gagarin's MiG, disrupting its flight.

Image caption Mr Leonov had previously hinted at the "other jet" theory in his book

"We knew that a Su-15 was scheduled to be tested that day, but it was supposed to be flying at the altitude of 10,000 metres or higher, not 450-500 metres. It was a violation of the flight procedure," he told the television channel.

He says Gagarin's plane went into a spiral at 750km/h following the close pass by the jet.

However, Mr Leonov declined to name the Sukhoi pilot.

"My guess would be that one of the reasons for covering up the truth was to hide the fact that there was such a lapse so close to Moscow," he explained.

The cosmonaut had already hinted in his 2004 book Two Sides of the Moon that a Sukhoi jet may have been flying below its minimum allowed altitude. Leonov had been flying a helicopter in the same area on the day of the accident and heard "two loud booms in the distance".

Many other theories have been advanced in the ensuing years, including one that a cabin air vent was accidentally left open in Gagarin's aircraft by the previous pilot. This, the theory claims, would have led to oxygen deprivation for the crew.

Mr Gagarin became the first person to journey into space on 12 April 1961, when his Vostok spacecraft completed a single orbit of Earth.