Democrat Rep George Miller, an advocate for worker safety in the apparel industry, has said he will not seek re-election for a 21st term in November.

Miller will retire later this year after 40 years in Congress covering issues including worker safety in foreign factories, education and health care legislation.

An investigator in defence of the health and safety of children and workers, Miller has successfully taken on asbestos executives, for-profit colleges, subsidised agribusiness, mining corporations, and oil companies.

He is also well-known for his work on long-term issues and engaging directly with people such as garment workers, most recently in Bangladesh.

The Nation magazine recently named him the 2013 'Most Valuable Member of the House' for his work on the minimum wage, food stamps, garment worker safety, and fair trade policy.

Of his departure, Miller said: "I'm proud of what I have been able to accomplish on behalf of children, working people and the environment, in my district and for our country. Now, I look forward to one last year in Congress fighting the good fight and then working in new venues on the issues that have inspired me.

He added: "I look forward to continuing to work on the issues that have been at the core of my career and my passions, in particular education reform, economic fairness, and improving labour standards in the United States and abroad, such as in the worldwide garment industry."