Just under a quarter of the letters are going to businesses within the marketing and communications industry.
The initiative, which follows PRWeek and the PRCA’s moves to draw attention to unfair treatment of interns in the PR industry, is accompanied by a government publicity drive aimed at young people.
While the PR industry has improved its record, it is understood that many of the employers HMRC is writing to come from the creative industries.
The letters warn that HMRC will be carrying out targeted checks – including office visits – to make sure they are paying all their workers the correct National Minimum Wage rate.
Employment relations minister Jo Swinson said the letters were just one example of the Government and HMRC working closely to clamp down on rogue employers.
Swinson will visit Edinburgh Napier University today to unveil a video and posters explaining to young people what their rights are in relation to the National Minimum Wage.
The advice is produced by the Department for Business, Innovation & Skills in collaboration with Channel 4 internship and apprenticeship scheme 4Talent.
Swinson said: "Anyone considered a worker under the law should be paid at least the National Minimum Wage, whether they are an intern, or someone on work experience… whether they are employed for a week or a day.
"Anyone who feels they are being exploited should contact the Pay and Work Rights Helpline on 0800 917 2368. Any intern calling that number will be prioritised by HMRC."
HMRC will also reveal that it has actively targeted employers in the fashion sector in the past year and identified two employers not complying with National Minimum Wage law.