The ex-fiancee of a British royal has been found guilty of a frenzied knife attack on a Dublin couple who were on a trip to London.

Zeaphena Badley (35) sliced through the back of student Oona McCabe's right knee with a rusty Stanley blade as she walked into Burger King on London Street, Paddington.

She then cut the 24-year-old's boyfriend, Kevin Cannon, who is from Walkinstown, under the chin when he tried to intervene, Southwark Crown Court heard.

Badley then launched herself at Ms McCabe a second time, repeatedly hacking at her head and leaving pools of blood on the fast-food outlet's floor.

Badley was once engaged to a godson of Prince Charles.

Stranger

The court heard that she had launched another unprovoked attack on a stranger on Edgware Road, just 36 hours earlier.

"She was arrested for that offence in the evening and bailed by the court on January 16 - the very day [before] where, at about 12.30am in the morning, she committed this assault," said prosecutor Tom Nicholson.

Badley is serving a 20-week sentence for that attack and has largely ignored the current proceedings.

A jury took less than an hour to find her guilty of causing GBH with intent to Ms McCabe.

She was also convicted of two counts of assault occasioning actual bodily harm against Mr Cannon and a passer-by.

Badley has 21 convictions for 36 offences including arson, theft and a string of assaults against members of the public and police dating back to 1999.

The judge, Mr Recorder Andrew Wales, QC, ordered psychiatric reports ahead of sentencing.

The former Kensington resident was engaged to Lord Mountbatten's great-grandson Nicholas Knatchbull, titled Lord Romsey, for 18 months, before the wedding was called off in 2013.

She was sleeping rough when she spotted the couple walking over to the Burger King following a night out in Soho.

"They had never seen each other before, they had never talked to each other," said the prosecutor.

"She was just effectively unlucky to be there at the time when the defendant chose to make her attack."

CCTV footage showed Badley slashing at Ms McCabe as they entered the restaurant before slicing Mr Cannon's face and dragging Ms McCabe to the ground, where she resumed her attack. The student told jurors that she needed 11 stitches to the front of her head, 13 to the back of her head, and one to her face following the attack, which left her covered in blood.

Badley could be heard on body-worn footage replying "brilliant" when being arrested.

During a brief appearance in court, before the jury arrived, she said: "I wasn't well."

She told the court "the medication was helping" now, before adding: "But I still hear voices."

However, she quickly grew increasingly aggressive when shown the CCTV of the attack for the first time.

After insisting "they're the real criminals" she then stormed out and refused to play any further part in the trial.

Tears

Ms McCabe and her boyfriend later gave evidence via video- link from Dublin.

The judge asked each of them: "If anyone were to suggest that the woman was acting in self-defence at any point in the incident, what would you say to that?"

Wiping away tears, Ms McCabe labelled the suggestion "ridiculous", while her boyfriend called it "crazy".

Badley, formerly of Cromwell Road, Kensington, London, denied grievous bodily harm with intent and two counts of assault occasioning actual bodily harm.

She remains in custody ahead of sentencing on a date to be set later.