The cut-price fashion chain said it was informed that the suppliers had subcontracted embroidery and sequin work on a small number of designs to unapproved subcontractors, which in some cases used children.
Primark has now cancelled all new orders with the factories and has stopped the sale of the embroidered garments already made.
The affected garments reportedly represent 0.04% of Primark's worldwide sourcing.
It was the BBC's 'Panorama' programme that uncovered the use of child labour, which led Primark to investigate. Panorama's findings are due to be broadcast next Monday.
Primark said it has a strict Supplier Code of Conduct, which expressly bans the use of child labour and does not permit suppliers to subcontract without specific consent and agreement.
A spokesperson for Primark said: "None of these suppliers is permitted to use home working.
"Under no circumstances would Primark ever knowingly permit such activities whether directly through its suppliers or through third party subcontractors.
"Primark does not tolerate serious breaches of its fundamental principles."
Primark said it has a system of audits that have so far proven "very robust" and "extremely effective" at raising supplier standards. Primark uses third party auditors, including SGS.
In addition to sacking the factories at fault, Primark has met with existing suppliers to reinforce its stringent trading standards and is set to appoint a non-government organisation in southern India as a partner to identify any unauthorised subcontracting.
Primark attributed its low prices to not overcharging customers.
In a statement, Primark said: "We are able to offer good value and good quality because of low markups and big volumes.
"We use simple designs, our overhead costs are extremely low and we don't run expensive advertising campaigns."
Primark is now the second biggest clothing retailer in the UK, in terms of volume, with nearly 170 stores.
Channel 4 pulled a potentially controversial documentary from its schedule at the start of this month, which involved examing the business practices of high street stores including Primark.
The show, called 'The Devil Wears Primark', looked at the working conditions in the foreign factories that supply cheap clothing.
It was reportedly pulled for editorial reasons but Channel 4 said they would air it at a later date.