Jeremy Corbyn was a "useful idiot" for Sinn Fein, the author of a hard-hitting new biography of the Labour leader has said.

But Tom Bower, writer of Dangerous Hero: Corbyn's Ruthless Plot For Power also says that Mr Corbyn's long history of support for physical force Irish republicanism is no longer a serious political liability for him as he tries to steer the Labour Party back to power.

The award-winning author and journalist, who has penned 23 books, including several acclaimed political biographies, worked for the BBC in Belfast in the 1980s. He is also a former journalist with the BBC's flagship Panorama current affairs programme.

Speaking to the Belfast Telegraph, Mr Bower (72) said Mr Corbyn was "frightfully partisan" about Ireland.

"He really wasn't interested in peace, he was only interested in uniting Ireland under a Sinn Fein government," he said.

"He was a Communist, and that suited him. He liked struggles against the Establishment.

"That's why he supported (Sinn Fein), because they were challenging the capitalist government.

"He never really tried to forge any relationship with Protestants, or even with the Republic.

"He seems never to have had any sympathy for the people who were victims, which I found very odd. I found that very unattractive about him."

The 350-page book charts Mr Corbyn's political career, including his decades-long support for Irish republicanism and its efforts for a united Ireland.

Mr Corbyn has previously faced questions over his meetings with Sinn Fein during the 1980s and 1990s and his views on the republican movement during that period.

Among the many Ireland-related issues Mr Corbyn engaged with over his career, Mr Bower pointed to one in particular in his book: how Mr Corbyn reacted when he learned of the Brighton bombing, when the IRA attempted to murder the British Cabinet.

Mr Bower wrote: "The destruction of Britain's industrial heartlands (by the Tories) made Corbyn's hatred of Thatcher personal.

"The proof was an early-morning phone call he received on 12 October 1984 from Val Veness (who looked after his Westminster office).

"An IRA bomb, she reported, planted at Brighton's Grand Hotel, had killed five Conservatives and injured 31 others, narrowly missing its main target, Margaret Thatcher.

"Corbyn, a man who promoted himself as a compassionate pacifist, made absolutely no comment to Veness, either of shock or sympathy. He made no statement welcoming Thatcher's emergence from the rubble, or praising her subsequent defiance."

Just two weeks later Mr Corbyn invited convicted IRA terrorists Linda Quigley and Gerry MacLochlainn into the House of Commons - to the fury of many MPs.

Speaking to this newspaper, Mr Bower also suggests that Mr Corbyn was not entirely trusted by his Irish associates.

He added: "The other thing which is quite interesting is that although he was close in the sense that he chaired meetings for (Martin) McGuinness and Gerry Adams, they clearly didn't trust him,

"They never told him that while he was chairing meetings or organising support for dead IRA activists, that they were actually negotiating peace with the British Government.

"In their view, he was a useful idiot. They didn't trust him at all. He knew nothing about the negotiations. They just liked that he was available to chair meetings as part of their protests."

Mr Bower said there was a contradiction at the heart of the MP's political persona. "There is a pattern of him being a pacifist, and always associating with terrorists. Not just in Ireland, but in Central America, South America and the Middle East.

"He has two faces. He is very good at putting on the face of a 'good bloke'. But actually he is quite a ruthless Marxist, who has spent a lot of time now getting rid of the moderate social democrats, which he also did when he was in Haringey.

"He's not somebody who actually cares for anyone who opposes him. He doesn't tolerate disagreement." Mr Bower said Mr Corbyn had copied the ruthlessness of his revolutionary heroes.

"He is a very stubborn Marxist Communist, who actually wants to change Britain into a Communist society, and will brook no opposition to that," he said.

Mr Bower said Mr Corbyn's pacifism "was always a cover", adding: "He believes that his enemy should be the pacifists, and the people he supports should be allowed to fight and kill for victory."

Despite his links to Northern Ireland politics, Mr Bower does not believe that Corbyn's history is now a liability in UK politics.

He added: "I think that in the election of 2017 the Tories and most like-minded people would have thought it was a great liability. But what they discovered was that amongst people who were under 50, it was all history.

"He was able to brush it off saying he was struggling to find a peace deal.

"But anyone who knew him knew that he had no interest in peace at all.

"My view now is that he is far more culpable as a Marxist, than over his activities in Ireland."

Turning to the events this week, when a series of Labour MPs resigned from the party, Mr Bower said the fracture was evidence that at least some at the highest levels of the party have had enough of Mr Corbyn's modus operandi.

He added: "What has happened is that this group of socialists, of Labour members who have now left the party, those MPs have recognised exactly what sort of man Corbyn is, and what he represents - and that's why they have finally decided to leave. They don't want to be associated with this hard-line, ruthless, intolerant Marxist."

Yesterday Mr Corbyn denied there is "wide scale" bullying in Labour as another MP quit the party complaining of anti-Semitism within its ranks. Former minister Ian Austin said he was "ashamed" of the party.

Dangerous Hero: Corbyn's Ruthless Plot For Power is published by William Collins (£20).

Belfast Telegraph