The CMI survey found that male managers were paid an average of 6,076 pounds a year more than their female counterparts, while male directors earned 49,233 pounds more.
Jenny Watson, who heads the Equal Opportunities Commission, said it was "alarming" that the gender gap was worsening almost 40 years after the Equal Pay Act came in.
She also called on the Government to bring in laws to close the gender gap once and for all.
The CMI, which carries out an annual survey of Britain's 4.6 million managers, contacted more than 42,000 people, ranging from management trainees to chief executives in public and private sector jobs.
It found that male managers were paid 12.2 per cent more than females in the year to January 2007; compared with 11.8 per cent the previous year. It is the first time in 11 years that the gap had widened.
At director level, men earned 23 per cent more, an increase of three percentage points. This was despite evidence that women were more motivated and productive, with 63.4 per cent of them earning bonuses, against 55.9 per cent of men, though their bonuses tended to be smaller, at 10.2 per cent of income, compared with 13.8 per cent for men.
A spokesman for Harriet Harman, the minister for women, said: "Much more needs to be done to tackle unequal pay."
In July, the Equal Opportunities Commission said in its annual report that the gender gap was "decades away" from being closed, with women in full-time jobs earning on average 17 per cent less than men.