Cardinal Keith O'Brien, who was forced to resign by the pope last week, has made a dramatic admission that he was guilty of sexual misconduct throughout his career in the Roman Catholic church.

In a short but far-reaching statement issued late on Sunday, the 74-year-old stated that "there have been times that my sexual conduct has fallen below the standards expected of me as a priest, archbishop and cardinal".

The former archbishop of St Andrews and Edinburgh, and until recently the most senior Catholic in Britain, apologised and asked for forgiveness from those he had "offended" and from the entire church.

O'Brien was forced to resign last week by Pope Benedict XVI, barely 36 hours after the Observer disclosed that three serving priests and a former priest were accusing him of "inappropriate acts" against them nearly 30 years ago, in a formal complaint to the pope's ambassador to the UK.

The cardinal had "contested" those allegations, while his officials said he was taking legal advice.

But now O'Brien has effectively admitted he had been breaching the church's strict rules on celibacy and its bar on homosexuality since he became a priest – and during his 10 years as a cardinal.

It was alleged that some of these incidents were "drunken fumblings". One case reported by the Observer involved repeated sexual contact.

On Friday, there were claims that complaints had been made to the nuncio, Archbishop Antonio Mennini, by a fifth priest last year, about an alleged incident in 2001.

Announcing that he would now retire entirely from public life and from the frontline duties for the church he once led, O'Brien said: "In recent days certain allegations which have been made against me have become public. Initially, their anonymous and non-specific nature led me to contest them.

"However, I wish to take this opportunity to admit that there have been times that my sexual conduct has fallen below the standards expected of me as a priest, archbishop and cardinal.

"To those I have offended I apologise and ask forgiveness. To the Catholic church and people of Scotland, I also apologise. I will now spend the rest of my life in retirement. I will play no further part in the public life of the Catholic church in Scotland."

His statement goes significantly further than the apology and partial admissions which he made in his resignation statement last week, when he said: "Looking back over my years of ministry, for any good I have been able to do, I thank God. For any failures, I apologise to all whom I have offended."

O'Brien's much wider admissions are a significant rebuff to some senior figures in the Scottish church who had repeatedly downplayed the allegations disclosed in the Observer, calling them unsubstantiated, non-specific and anonymous.

The cardinal's office warned the Observer it faced legal action after it first contacted him. In further disclosures this weekend, the Observer reported that one complainant had alleged: "He started fondling my body, kissing me and telling me how special I was to him and how much he loved me."

In a fresh interview with the Observer, the former priest, who made his complaint to the nuncio in early February, said that after his disclosures he sensed "the cold disapproval of the church hierarchy for daring to break ranks. I feel [that] if they could crush me, they would."

O'Brien's resignation was remarkable in its speed; his apology is all but unprecedented in its frankness. Many sexual scandals or allegations of misconduct against individuals or the wider church have dragged on for years.

The cardinal was forced out only three days before the pope retired last Thursday. There is growing speculation that the Vatican acted swiftly because O'Brien had challenged one of the church's greatest orthodoxies – saying, in a BBC interview and only two days before the Observer story was published, that priests ought to be allowed to marry and have children.

Catherine Pepinster, editor of the weekly Catholic newspaper the Tablet, said Benedict and his close aides may have been extremely irritated because O'Brien had promised to renounce his once liberal views on some church teachings when he became a cardinal in 2003.

On becoming a cardinal the Vatican had made him swear an oath to uphold the teachings of the church, binding him to uphold its orthodox positions. He then took a hardline stance against gay issues and abortions.

Pepinster said his new statement would, however, allow the Scottish church to move on. "This is a shocking admission, but one that is in many ways welcome, not least because it seems Cardinal O'Brien must have been leading a double life, and that is now at an end.

"That must surely be a relief to him and a burden lifted. But it must also be a relief to Catholics in Scotland. The boil has been lanced, and it's time to move on. Too many scandals in the Catholic church drag on and on, but this one has been dealt with speedily, and a line can be drawn."