A mother-of-two has lashed out at Heathrow Airport after she was forced to throw out around 15 litres of breast milk she was trying to take onto a plane.

Terminal five airport staff allegedly made Jessica Coakley Martinez, of California, pour away the liquid milk and also dump an almost nine-litre frozen block of breast milk she had been accumulating to feed her eight-month-old son.

Ms Coakley Martinez took to Facebook to pen an angry open letter to the airport, saying she felt "completely defeated as a professional and as a mother" when staff made her "dump out nearly two weeks worth of food for my son."

The letter, which has been shared more than 4000 times, explains how Ms Coakley Martinez worked hard to collect the breast milk by pumping in aeroplane bathrooms, conference rooms and even closets.

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She went into detail about the "exhausting" efforts needed to succeed in her career and as a mother, saying some business trips covered eight cities in just 15 days.

The limit for liquids in carry-on luggage is 100mm, according to Heathrow Airport.

In her post, Ms Coakley Martinez acknowledged she "should have looked up the civil aviation rule", but said the ban on women from taking breastmilk onto a plane if they weren't travelling with their child was "incredibly unfair and exclusionary" to working mothers like herself.

"That being said, more than 300oz (8.9L) of that milk was frozen. Solid. Like a rock. I was willing to let go of the liquid milk," she wrote.

"But you also wanted the solid milk because it could 'melt and become a liquid'."

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Ms Coakley Martinez said she tried to argue with airport staff to no avail.

"If I acted irate, it's because it was the only appropriate reaction I could muster," she said.

"Security is the priority, but it isn't and shouldn't punish those you intend to protect. Beyond literally taking food from my child's mouth, you humiliated me and made me feel completely defeated as a professional and a mother.

"I hope the next time you encounter another mom just trying to make it work and looking for a little help along the way, you consult your conscience (as well as a physical science textbook) and reconsider your options."

Airport officials reportedly confiscated the milk as a "non-compliant item".

The furore comes as Jetstar offered an apology to an Australian mother who said a flight attendant told her to cover up while breastfeeding her child on a flight from Brisbane to Townsville.

The mother, Natalie Jane Sawyer, posted on Facebook that she was given a back row to herself to express while passengers were seated and the seatbelt signs were on, but the crew member requested she cover-up because people would soon be visiting the toilets.

While she admitted the attendant asked nicely, the Queensland mother-of-three said she was wearing a breast-pumping bra and a shirt was covering her pumps, so she wasn't exposed.

"It shouldnt (sic) matter anyway if I was covered because legally I have a right to pump anywhere, anytime I want and if I feel the need to cover up I will do so and not because people have to walk down the aisle to the toilet," she posted.

A Jetstar spokesman said the company attempted to contact Ms Sawyer on Monday afternoon to pass on their apologies.

"We want all mothers to feel welcome and respected when breastfeeding on-board our flights and deeply regret the offence caused by our crew member's remarks," the spokesman said.

"We'll work with our crew and remind them of our breastfeeding policy to prevent this from happening to other mothers."

-Additional reporting from AAP