The statement said breastfeeding has multiple health benefits for children as it boosts their immune systems and provides key nutrients for development.
It celebrated a leap from 10% to 48% globally in the number of children who were exclusively breastfed in the past decade.
However, it emphasised that supportive workplaces are the key to achieving the global 2030 target of 70% of children being breastfed.
The organisations suggested policies such as paid maternity leave, breastfeeding breaks, and a room where mothers can breastfeed or express milk.
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The letter said: “These polices generate economic returns by reducing maternity-related absenteeism, increasing the retention of female workers, and reducing the costs of hiring and training new staff.”
Nicola Wallbank, employment law partner at law firm Freeths, said employers have health and safety obligations towards breastfeeding mothers.
The Health and Safety Executive (HSE) recommends that employers make an individual risk assessment for breastfeeding workers, considering if their work presents an extra risk for the mother and their children, for example by exposing them to organic mercury, radioactive material or lead.
Speaking to HR magazine, Wallbank added: “It is important to remember that breastfeeding covers both breastfeeding a baby and expressing milk.
“Employers need to provide suitable facilities for breastfeeding mothers that include somewhere to lie down, and which are hygienic and private so that mothers can express milk if they choose to.
“They should also have access to facilities to safely store their milk, for example in a fridge. HSE guidance is clear that toilets are not a suitable place for breastfeeding or expressing.”
Wallbank said the law does not strictly require employers to give paid breaks for breastfeeding, but not doing so is legally risky.
She said: “If an employer refuses to allow a woman the flexibility she needs to express milk or to breastfeed this may amount to indirect sex discrimination unless an employer can objectively justify its approach.
“There is also case law to support the argument that forcing an employee to express milk in the toilets or in their car can amount to harassment on the grounds of sex, where it creates a degrading and humiliating environment for the employee.”
The key for employers to avoid this is creating a supportive and accepting culture, Wallbank said.
She added: “It is important to ensure there is a culture of acceptance of breastfeeding, that facilities are got right, and that any banter that is offensive or humiliating is not tolerated.
“Returning to work after maternity leave can be a challenging time for women. Discovering you have no or poor facilities to support you to continue to breastfeed can be demoralising and taint your return.”
HR teams should also consult employees to understand how they would prefer to express milk or breastfeed in work, according to Meg Murray Jones, founder of postpartum care provider, Postpartum Plan.
Speaking to HR magazine, she said: "I speak to a lot of HR professionals as I offer training in postpartum support for employees. The one thing I ask is 'do you ever just ask what your employees actually need?'.
“This might sound simple, yet so many HR professionals have been tied up in policy rather than individual needs. Some people may want a breastfeeding room, others would hate that idea. Some may want to express milk whilst others can't express at all.”
