Police believe a serial date rapist who was arrested four times over a 16-year period before being charged may have attacked other women.

Brian Witty was found guilty on Tuesday of three counts of rape and one of sexual assault of women he had picked up on dating websites and in bars.

But campaigners have asked why it took so long for the former Territorial Army captain to be stopped.

In 1995, 2006 and 2008, the ex-Parachute Regiment member's victims reported him to police within hours and he was arrested. But each time he insisted the sex or contact had been consensual, and the Crown Prosecution Service (CPS) advised against charging him.

It was only when his fourth victim alleged rape, in August last year, that the previous cases were reopened and charges brought.

The CPS said that while its reviewing lawyer in 2008 knew about the previous allegations, they could not say for certain whether that was the case in 2006.

"The prosecution said that failure to charge earlier had emboldened him and made him feel 'untouchable'," the group Women Against Rape said.

Detective Inspector Michael Murfin, of the Met's Sapphire unit, said Witty, 41, was a predatory rapist, and urged anyone who believed they had been raped or assaulted by him to contact police.

Witty was told he faced a long jail term. The five-week trial at Kingston crown court heard he switched between being charming one minute and a threatening bully the next, warning one woman he had been in the forces and could do whatever he wanted ? "even kill her". When another rejected his advances and began to cry, he said: "I don't believe this. I'm a good-looking bloke." He then told her: "When you say no, you mean yes."

Witty met two of his victims on the website datingdirect.com, raping one last year after she went back to his flat in Teddington, south-west London, following a date. He sexually assaulted the other in an alleyway after drinks in a Covent Garden bar, and raped both his other victims in his flat.

"Witty could not have believed that they were consenting, not while they were crying or pleading or begging him not to or telling him no again and again," the prosecutor Edmund Gritt told the court.

Women Against Rape's Lisa Longstaff said: "The trial has shown, once again, that with serial rapists police and CPS have a serial response ? they believe the rapist rather than his victim, and the way the investigation is handled is determined by that.

"The fourth investigation was far better than the previous ones. Witnesses who had been neglected were interviewed and evidence gathered."

The trial heard that police did not try to contact other women Witty had met on the dating site, or investigate whether he had been telling the truth when he said in three of the cases that he was about to go away.

The head of the CPS in London, Alison Saunders, rejected the claim that prosecutors believed alleged rapists over victims, saying the earlier cases had not gone ahead because they had determined there was insufficient evidence for a realistic prospect of conviction.

She added: "The decisions to not charge the three previous allegations were made at a time when our specialist knowledge of the complexity of rape cases was not at the high level it is today."

In a statement, the Met said it was not in a position to discuss specific actions relating to the earlier investigations. "However, Sapphire moved under the command of the Specialist Crime Directorate in September 2009, with the aim of providing a more consistent service to victims whose care and wellbeing is at the heart of each investigation," it said. The new single command structure meant better-trained and more experienced staff and improved, centralised intelligence, the statement added.

Murfin said: "We want victims to have the confidence that we are here for them, will believe them and will conduct our investigations professionally."