William Bulger, 69, the former Democratic leader of the Massachusetts senate, has been a formidable presence in the state's political and civic life for several decades. But a series of unanswered questions and unsatisfactory answers regarding his relationship with his elder brother, James "Whitey" Bulger, a mob figure linked to 21 murders, has forced him to stand down.

There have been persistent rumours that "Whitey" Bulger is hiding in Britain and Scotland Yard has been investigating sightings of him in central London.

Before going on the run nine years ago, he was one of Boston's most feared criminals. As boss of the Winter Hill gang, which controlled the city's drug traffic for several years, he was implicated in a great deal of other illegal activity.

He also allegedly became a prized informant for the FBI and fled just before his indictment on federal charges relating to the murders, after a tip-off from one of his handlers. Court testimony suggests he corrupted many of his handlers, buying them off in return for tips about when other mobsters were informing on him.

William Bulger stepped down as president of the University of Massachusetts on Wednesday after pressure from the state's Republican governor, Mitt Romney, and the attorney general, blaming what he called "a calculated political assault".

The resignation marks the end of a protracted and vicious battle which consumed Boston's political elite, with everyone from Senator Teddy Kennedy to the Boston Globe taking sides.

Mr Romney threatened to eliminate the post of university president rather than allow Mr Bulger to continue in the role.

But Mr Bulger retained the full backing of the university until the end, including a severance package of just under $1m (£620,000) and a standing ovation on acceptance of his resignation on Wednesday . He also hangs on to the largest pension in the history of the Massachusetts government, at $250,000 a year.

But despite the support, his political fortunes started to decline after he appeared to be evasive under oath about his connections with his brother. Transcripts of his testimony before a federal grand jury were leaked, in which he said he would never help to apprehend James.

He later pleaded the fifth amendment, the right to remain silent for fear that speaking might incriminate himself, before a congressional panel investigating the FBI's ties to its mob informants. He only appeared before the panel after he had been granted immunity, but then appeared to have either forgotten vital information or allegedly gave false testimony.

Among other things, Mr Bulger claimed that he had only learned of his brother's criminal activities through newspaper accounts, that he did not recognise the name White Hill gang, that nobody from the FBI had ever asked him where his brother was, and that he had believed his brother's crimes were confined to run ning a gambling operation. He admitted he had spoken to the fugitive once on the telephone, but insisted he had no idea where he was.

Since most of Massachusetts had heard of the White Hill gang and knew his brother's activities extended well beyond gambling, few believed that William Bulger was telling everything he knew.

Moreover, FBI agents went to Mr Bulger's office not long after his brother fled but were turned away, according to bureau reports.

The Bulgers' younger brother, John, lost his state pension in May when he was convicted of perjury after lying to a federal grand jury about his knowledge of a safety deposit box owned by James.

The men's varied life paths date back to their childhood in a south Boston housing estate. While "Whitey" fell into crime, William became a classics scholar - a path that guided his belief in the difference education could make in people's lives.

But he became the focus of Mr Romney's desire to dismantle the powerful Massachusetts Democratic party machine, and his resignation is seen as a major victory for the governor.

"It clearly deteriorated into a political game," the university's pollster, Lou DiNatale, told the Los Angeles Times.

"I don't know if there's anything that Bill Bulger could have done in the 40 years of his really remarkable career to postpone this moment, because his brother was on a rollercoaster to hell. His fate was always tethered to his brothers.

"He spent his entire life trying to overcome that fate only to meet it."