Further, over half said there are not enough women in senior positions in their firm, and almost two third (63 percent) said all the decision-makers in their company are men.
There has been clear progress since Lord Davies' 2011 'Women on Boards' review, which made recommendations to improve boardroom diversity.
As of last year, all FTSE 100 firms have at least one woman on their boards.
However, firms have been criticised for placing women in non-executive positions to fill its diversity quota.
In a bid to rectify this, and get more women in permanent senior-level positions, O2 has partnered with CIPD -- the body for human resources and professional development - to create a guide for firms to support talented women in their organisation, "so that they are able to reach the highest levels without the need for artificial quotas", said Ann Pickering, O2's HR director and board member.
"Our research shows that, while the diversity debate has moved on outside of the office, not enough women are actually seeing this progress at work," Pickering added.
"We can't just look at women already at the top; we need to focus our efforts on women at every level, creating a strong pipeline of female talent across British businesses. If we fail to do this, there is a very real risk that these women will seek these opportunities elsewhere."
Dianah Worman OBE, public policy adviser for diversity at the CIPD, is calling for political parties in the upcoming election to commit to a new voluntary diversity target.
She said: "While there's been genuine progress towards government targets to improve boardroom diversity, too much of this has been skewed towards non-executive positions. We're calling on all parties in the forthcoming election to commit to a new voluntary target for at least 20 percent of executive director positions in FTSE 100 firms to be filled by women by 2020. This will encourage organisations to set a clear example of diverse leadership, and grant employees the benefits of management diversity from those actively involved in the business."
The guide, named 'Breaking the Boardroom: A guide for British businesses on how to support female leaders of the future', includes the opinions of over 2,000 UK women in business and insight from the CIPD and O2. It can be downloaded here.