Even more shocking than the barbarity of the terrorist attacks in London and Boston is the denial by authorities on both sides of the Atlantic about their common religious motive.

British Muslims offered the same explanation as American Muslims for slaughtering their fellow citizens — Islamic holy war — yet the political classes on both sides of the pond rejected it out of hand.

Just minutes after British-born Michael Adebolajo and another Brit ambushed and decapitated soldier Lee Rigby, 25, in the middle of a London street, he spoke calmly into a camera, his blood-stained hands still holding the meat cleaver: “The only reason we have killed this man today is because Muslims are dying daily by British soldiers, and this British soldier is eye-for-an-eye and tooth-for-a-tooth.”

Adebolajo, 28, added that their holy book commands them to seek such revenge.

London’s mayor dismissed his theological rationale as “deluded” and “deranged.” Prime Minister David Cameron said the sickening attack was “a betrayal of Islam.”

Added the British leader: “There is nothing in Islam that justifies this truly dreadful act.”

A month earlier and more than 3,000 miles away, however, naturalized US citizen Dzhokhar Tsarnaev offered the same justification for massacreing fellow Americans at the Boston marathon with his older brother.

In a note on the boat he was hiding in as police closed in, he called the Boston victims “collateral damage” in a holy war against the West. He also scrawled: “When you attack one Muslim, you attack all Muslims.”

Like his British counterparts, President Obama refuses to see the religious connection. He called the War on Terror over, simply because we say so — without acknowledging that Muslim terrorists certainly don’t think so.

“This ideology is based on a lie,” he insisted Thursday in a national security speech. “This ideology is rejected by the vast majority of Muslims.”

Is it though?

While such violence is written off in official circles as a distortion of the tenets of Islam, it is in fact scripturally supported. And jihad in defense of Muslims and Islam is actually mainstream thinking inside the Muslim community, widely preached in mosques and Islamic literature across Britain and America. According to recent surveys, moreover, the vast majority of Muslim-Americans harbor a similarly deep resentment against the US-led war on terror and the military waging it.

“Jihad and martyrdom attacks have doctrinal threads that trace back from the crime scene to core Islamic texts,” FBI counterterrorism analyst Bill Gawthrop said. Virtually every homegrown terrorism case since 9/11 cites jihad as motivation.

“Those threads are not fully, or widely, understood or accepted from a law enforcement, intelligence or political perspective,” he added.

Why? Western leaders have not read Islamic texts for themselves, relying instead on Islamic clerics and apologists to tell them what they say about jihad and other sanctioned violence.

That’s a problem. After the London beheading, for instance, Muslim leaders insisted the Koran forbids killing. With typical duplicity, they cited a portion of a verse — “Whoever killed a human being . . . it shall be regarded as having killed all mankind” — while leaving out the critical part: “ . . . except as punishment for murder or other villainy in the land.”

Villainy is defined in the next verse as “those who wage war against Allah,” a crime punishable by — you guessed it — beheading.

An almost willful blindness to what motivates the enemy is preventing the West from effectively defeating it, says retired Army Maj. Stephen Coughlin, who has advised the Pentagon on Islamic military doctrine.

“It doesn’t matter if it’s right or wrong, as long as the enemy thinks it’s right,” he said. “If the enemy says he’s fighting in the name of green cheese, then we’ve got to know green cheese!”

According to the Koran and other sacred Islamic texts, physical fighting is obligatory on every able-bodied male when Muslim lands are invaded by kaffirs, or non-Muslims. This meaning is undisputed by the four major schools of Sunni jurisprudence.

Jihad is also a communal obligation that can be waged by all other Muslims through the pen, tongue or purse. A share of all Muslim tithing, in fact, is set aside for jihad, Gawthrop points out.

The word “jihad” is derived from the Arabic term for warriors — “mujahideen” — who in the Koran are promised a higher reward in the “gardens of Paradise” than “those who sit at home.” (The London beheader’s nickname was “Mujahid.”)

Obama asserted that “Muslims are a fundamental part of the American family.”

But polls show their loyalties are divided between this nation and the greater Muslim world or brotherhood, known as “the Ummah.” A 2011 Gallup poll said Muslims here “strongly identify” with the global Ummah, which includes Afghanistan and Iraq and all other Muslim lands and therefore transcends nationalism, patriotism and even secular friendships in importance.

The same poll found that 83% of Muslim-Americans disapprove of the war in Iraq and almost half say the US never should have sent troops into Afghanistan — even after 9/11.

An earlier survey by Pew, moreover, found that 1 in 4 young US Muslims think suicide bombings to defend Islam are justified.

More and more Muslims living among us are taking violent jihad seriously. To them, the war isn’t just in Afghanistan and Iraq. It’s right here, and the war zones include not just sporting events and city streets but shopping malls, amusement parks, even schools — all fair targets in their holy war.

“You people will never be safe,” Adebolajo warned. “It will be the average guy like you and your children” — who are attacked.

Politically taboo or not, it’s time America had a frank national discussion about the “holy” words that motivate such bloodlust, and what they actually mean to the Muslim community, because it’s clear these words can kill.