Khadija Shah, 25, was on her way back home to Birmingham with her two children when she was arrested by police. They discovered 140lbs of the drug hidden inside clothes in three suitcases.
She claimed she took the bags as a favour for men she met in Pakistan and did not know what was in them, the Daily Mail reported.
The haul was thought to be the biggest amount of the class A drug ever uncovered at Islamabad airport destined for Britain.
In Pakistan, anyone arrested with drugs weighing more than 22lb (10kg) faces at least life imprisonment and at most the death penalty.
Miss Shah had been on holiday for a month and a half in the country, where she had been visiting relatives.
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Her boyfriend, Amar Ali, who plans to travel to Pakistan to pick up the children - who are from a previous relationship - said she had "stupidly" agreed to take the bags.
He told the Mail: "I told her not to do that. I told her to trust no one. When I was there recently everyone there wants to be your friend because you're from England.
"People try to get you to take bags to the UK for them. I said, 'Make sure you don't take any luggage for anyone'."
After her arrest yesterday, it emerged that Miss Shah had been jailed in England in 2010 for her role in stealing the life savings of an elderly woman with terminal cancer.