Its share of the total value clothing market will hit 18.4% this year compared to 14.3% in 2006.
Primark's success will help buck overall sales of value clothing by 8.9% on last year, retail analysts Verdict predict.
The chain will be the UK's number one value clothing retailer this year followed by George at Asda, Tesco, New Look and TK Maxx, according to Verdict's UK Value Clothing Retailers 2007 report.
This year alone, budget fashion sales will hit £8.5 billion and account for nearly 25% of all clothes bought in the UK.
Verdict lead analyst Maureen Hinton said budget fashion chains had made other clothes retailers push their prices down in order to compete.
"This trend heralds a new cycle in the clothing sector in the UK, where retailers must work much harder to inspire people to buy," she said.
Primark is famous for its 'fast fashion', which is when catwalk trends are quickly turned into more cheaply priced clothes for the high street.