A SECRET police video which helped convict a woman of murdering her "gentle giant" of a husband should never have been shown to the jury at her trial, the Court of Appeal was told yesterday.

Lawyers for Christina Button and her co-accused - her nephew, Simon Tannahill - said their central submission was that a covert recording of the pair, made at a police station, which the prosecution claimed implicated them in the killing of George Button, was unlawful and a breach of their right to a fair hearing.

Three appeal judges in London were yesterday urged to quash their convictions as unsafe because the video evidence had been obtained in contravention of the right to privacy under Article 8 of the European Convention on Human Rights, because they were unaware they were being filmed.

The prosecution maintained the evidence was rightly admitted on public interest grounds.

Button, 33, and Tannahill, 21, were jailed for life in December 2003 at Newcastle Crown Court for what trial judge Mr Justice Royce described as a cold and calculated killing.

The jury was told that Button persuaded Tannahill to murder Mr Button as the 53-year-old walked his border collie dog near their home in St Mary's Drive, West Rainton, near Durham City, in March 2003.

She was said to stand to profit handsomely from her husband's death by way of an insurance payout.

Mr Button was battered about the head at least six times and died in hospital two days later.

His wife had run up debts of nearly £200,000 and the couple had remortgaged their home to pay off money she owed as a result of "compulsive spending".

Yesterday, defence counsel Alistair Webster QC argued that the video tape was highly damaging.

He said that in a case which hinged on circumstantial evidence, such "prejudicial material" was bound to have had an impact on the jury.

Addressing Lord Justice Tuckey, Mr Justice Leveson and Sir Charles Mantell, he said: "In so far as it is evidence which should not have been placed before the jury at all, its admission rendered the conviction unsafe."

Button's lawyers also say the jury in the original trial were wrongly allowed to hear several key disputed pieces of evidence, including testimony about a past theft by Button from her employers and evidence from neighbours about her lavish spending habits.

The court reserved judgement on the appeal.