The father of dismembered British girl Lucie Blackman has criticised Japanese prosecutors after the millionaire property developer suspected of the killing was today acquitted.

Joji Obara was today found not guilty of raping and killing Lucie in 2000, in a surprise verdict that has come as a devastating blow for her family and a grave embarrassment for Tokyo justice authorities.

Obara received a life sentence in prison, nonetheless, after guilty verdicts on eight other charges of rape and one of raping and killing another Tokyo bar hostess, Carita Ridgway, a 21-year-old Australian. His legal team immediately said that Obara would appeal.

Lucie's family were appalled by the judge's ruling. Jane Steare, her mother, said: "I'm heartbroken, absolutely heartbroken. I just can't believe this verdict. My worst fears have come true."

She said that she was now looking at how to reverse the judge's decision. "As Lucie’s mother and the sole legal representative of my daughter’s estate, I will be consulting the authorities and my legal advisers to understand what options are available to me in my continuing search for justice."

Mrs Steare did not fly to Japan for the conclusion of the six and a half year trial, saying that she was unable to face seeing Obara in court.

"There is nothing to prove that he was involved in the rape and her death," the chief judge, Tsutomu Tochigi, told Tokyo District Court.

"The court cannot prove that he single-handedly was involved in her death. What is clear is that the victim was together with the accused and then vanished and next was found dead."

Lucie’s father and sister, Tim and Sophie - who had to have the proceedings explained to them by British Embassy officials, with whispered translations - were escorted out of court during the lunch interval.

Later, Mr Blackman criticised the way that prosecutors handled the case, saying they had taken an interminable time to bring the case to court, failed to lead what he considered important evidence, and were now speaking of today's verdict as just the first round in a battle that would go to Japan's higher courts.

"When we came out of the courtroom we were quite pragmatic about the sentence... but following our meeting with the prosecutors we feel very much more upset about this and we feel that Lucie has been robbed of justice," he said.

"One piece of evidence came up (in Obara's favour) that we thought had been countered by other matters, so we asked the prosecutors, and they said that that evidence hadn't been put to the court," he said. "It was an extraordinary moment."

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