Sentence for Queen portrait defacer

A fathers' rights campaigner is facing sentencing in London for defacing an Australian artist's portrait of the Queen with paint in Westminster Abbey.

Tim Haries, who told jurors he vandalised the picture to highlight the "social justice issue of our time", had denied a charge of causing criminal damage of more than STG5000 ($A9,185) but was found guilty at London's Southwark Crown Court last month.

He will be sentenced on Wednesday.

The father-of-two smuggled a can of purple spray paint into the abbey on June 13 last year before writing the word "help" on the painting by Australian artist Ralph Heimans, which is worth STG160,000 ($A293,900).

Jurors heard how Haries shouted "fathers for justice" as he scrawled the graffiti on the large oil painting before being tackled to the ground by steward Peter Crook.

He told Mr Crook: "Sorry mate, I've got nothing against the Queen," before telling a police officer who arrived at the scene that he was "guilty as charged".

Photographs of the incident were posted on a Facebook page for the Fathers4Justice group.

Haries, 42, decided to represent himself towards the end of his trial and directly addressed jurors, telling them he carried out the act as a protest against the "social catastrophe" of fathers not being allowed access to their children.

He said that, while he had nothing against the Queen personally, he targeted her portrait because of her symbolic role as head of the justice system.

Haries later said he "would not hesitate to petition the Queen again by peaceful protest on behalf of my children and the millions of children separated from their fathers by the British Government".

The portrait by artist Ralph Heimans was cordoned off by a rope in the Abbey's Chapter House as part of a wider exhibition to celebrate the Queen's Diamond Jubilee.

The 3.35m by 2.7m painting was bought by Westminster Abbey after being on display in Australia. It cost STG7300 ($A13,410) to repair.