Clad in an ill-fitting suit and heels, a middle-aged woman with blonde dreadlocks emerged from the Lagos flight, dwarfed by Nigerian businessmen. Gingerly, she stepped past a springer spaniel enthusiastically darting between passengers in the hope of discovering drugs.

But as she rounded the corner, a customs officer appeared at her shoulder. Wide-eyed, the 47-year-old explained that she was a care assistant on her first trip abroad, who had paid more than £1,000 for a ticket to Africa only to spend four days there.

To UK Border Agency officials looking for any anomalies that might indicate a drugs mule, her story appeared as incongruous as her outfit. Her bags were searched. Nearby, another official took swabs from the luggage of a businessman wearing designer sunglasses. As the wipe passed through the detection machine its screen flashed red: traces of cocaine had been found. But no such drugs were discovered on the woman's luggage and she was asked to submit to a full body scan in case she was one of countless mules who enter Britain each year, their stomachs stuffed with parcels of drugs.

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These days, an increasing number of middle-aged and elderly women are being used by smugglers. In a worrying trend, grandmothers, often with serious health problems, are being locked up in British jails after agreeing to carry cocaine in suitcases or risking death by swallowing drug-filled "wraps".

Only a few weeks ago, a British grandmother, Ambrozine Heron, 77, was jailed for 13 years for trying to smuggle almost £1m worth of cocaine into the country in her mobility vehicle. Customs officers at Dover found 16kg of the drug in Heron's specially adapted Nissan Pathfinder after she arrived from France. The pensioner had suffered a stroke and had diabetes, asthma and hypertension. Her daughter, Paulette Chambers, 49, was jailed with her.

"I don't know anything about that," she told police. "I just went with my daughter. I didn't ask any questions because I'm not a nosy person. I don't smoke or drink. What am I doing with drugs?"

Hibiscus, a charity that has won widespread praise for tackling the issue of drug mules, insisted it had seen a dramatic rise – as much as 40 per cent – in women in their late sixties and seventies being conned into carrying or swallowing cocaine. Often, they are Nigerians or Ghanaians, and most are first-time offenders who have left their children behind. Many are so sick that they console themselves they will at least get health care if caught. Each jailed offender costs the UK taxpayer an average of £130,000 before health costs are added.

"I am convinced that is a lot of the motivation," said Olga Heaven, director of Hibiscus. "Ninety per cent of the Nigerians have high blood pressure and cholesterol as well as gynaecological problems. We have one case of a woman in her early fifties with a brain tumour who has lost her eyesight."

At one point, as many as 40 drugs mules were coming in on flights from Jamaica but a two-pronged attack of increased law enforcement and education has solved the problem, with just one mule from the island caught last year. But the trade has simply shifted to Nigeria and Ghana, and other African nations as well as smaller Caribbean islands.

In February, a Nigerian mother with five children, Abosede Fehintola, 57, was jailed for eight and a half years for trying to smuggle £500,000 of cocaine through Stansted in her suitcase. Between 2001 and 2006, the number of Nigerian women in British custody rose more than ten-fold and the number of men trebled. Figures for Ghanaian prisoners doubled during the same period.

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"These older women are more naive and less educated about the consequences than young people," Ms Heaven said. "We used to see a lot of women who wanted to go to university but now they know the risks are too great. The older women are more desperate and have health issues. We have one woman of nearly 73 who was given a 12-year sentence [for smuggling 6kg of cocaine]. She looks like she is 80 and she wanted to see her grandchildren in this country. She is going to die here."

She added that they had recently seen a Nigerian women in her sixties who tried to smuggle 8kg of drugs into the country in a wheelchair. "They don't know any better and they are desperate. They are not educated. Every one of them is a victim."

Gigures released by HM Revenue & Customs show that 3,120kg of cocaine were seized by last year, an increase of more than 20 per cent on 2006-07. The Serious Organised Crime Agency (Soca), along with a variety of other agencies, seized a total of 90,000kg of Class A drugs in 2007-08. But an estimated 40,000kg cocaine slips past them. Much of it is brought in by shipments or freight,

For the past five years, the UK has been the "cocaine capital of Europe". The illicit drugs market in the UK is worth up to £6.6bn, with cocaine now the second most popular drug, after cannabis, with 800,000 estimated users. Much of it is now heavily cut with other chemicals, and prices vary from £30 to £45 per gram, according to purity.

Mules are being forced to swallow larger amounts, as much as a total of 1kg in condom-wrapped balls, said Ms Heaven. "That shows how desperate they are to put their body at such risk. They have no access to social welfare. A child might have taken ill or the mother can't pay the school fees. Someone will lend her money. When she can't pay it back they apply a kind of duress."

Tackling the problem is multi-pronged. While the UK Border Agency – a new agency that has customs and immigration working in tandem – and SOCA are mentoring officials in Africa, Hibiscus has been operating a successful programme of warning women in Ghana about the dangers of smuggling, which has led to fewer arrests. Just two weeks ago the charity started a new poster campaign in Nigeria entitled "Maame goes to London", a cartoon depicting a widow who turns to smuggling to feed her family, only to be imprisoned in the UK for 14 years as her children die of starvation.

Shola Shokumbi is a perfect example of the women caught. At the age of 52, she swallowed 116 wraps – 1.5kg of cocaine – and is now serving a seven-year sentence in Downview Prison. Her children are being cared for by her 83-year-old mother in Nigeria and no longer go to school. "I don't have enough time to talk to them on the phone because they are always crying," said Ms Shokumbi, whose real name has been withheld in case the drugs gangs will target her family for the cocaine she never delivered. "I am begging the British people for mercy. We are all mothers. Send us back home because of our children, so they can grow up good, useful people."

Back at Heathrow, no drugs were found on the woman in the cheap suit. Her story was one of innocence. Customs officials insist they are not looking for any particular profile. But, with 60 million passengers passing through the world's busiest international airport each year, they concede "it is like looking for a needle in a haystack".

The mule's story 'I have learned my lesson'

*Madeleine Smith looks like an average secretary. Softly spoken and polite, she does not look like a convicted drug smuggler. But that is exactly what the 42-year-old mother of a small boy was. The product of a church-going family from Jamaica, Ms Smith (not her real name) lived in Croydon with her partner, a teacher, and took her two-year-old son to visit relatives in the Caribbean. She was in a wine bar when she began chatting to a striking, smart stranger. "He looked like a normal guy, decent, not scruffy. I was bragging that I was not a tourist, I knew my way around and he must have thought, 'Here's my target'. I was such a silly cow." Then the man's mood changed and he threatened her family if she did not take a parcel of "men's toiletries" back to the UK. "He took out a knife. It had an embroidered handle. I didn't tell anybody." At Manchester airport, customs found 2kg of cocaine. Ms Smith pleaded guilty and served three years of a six-year sentence. "Prison was horrid. At first, I slept in my coat because it was so cold and damp. The women were vulgar, loud. At night, you'd hear them banging on the walls and screaming. They were coming off drugs." Ms Smith has been reunited with her son but separated from her partner. She now works full time for a charity. "I have learnt my lesson," she said.

Terri Judd