McDonagh asked David Cameron during Prime Minister's questions to ensure next week's budget includes regulation to ensure no worker is paid less because of the introduction fo the National Living Wage.
McDonagh told Cameron: "A friend of mine works a 39 hour week including Sundays and Bank Holidays on the shop floor at B&Q.
"To introduce [the living wage], B&Q are cutting allowances. As a result he's going to take home £50 a week less or £2,600 a year after the hourly rate goes up."
A B&Q representative said they were offering compensation to the equivalent of 12 months of any reduction in overall pay for anyone who suffers a wage drop and that no employee's basic salary would be hit by introducing the National Living Wage.
"From 1 April 2016 our minimum rate will be £7.66 per hour (£15,524.48 pa), 46p more than the National Living Wage’s £7.20 per hour", said B&Q.
"This will be our minimum rate for all employees, regardless of age, not just those 25 years and over who are covered by the National Living Wage."
B&Q said 90 per cent of customer advisors would get an increase in their basic salary on 1 April 2016.
The company said they were"aligning allowances" which they said were paid inconsistently which includes standardising bank holiday pay and bringing Sunday pay in line with other trading days.