More than four out of 10 men will forego their right to extended paternity rights, according to a new survey.
Of the 1,000 men polled by price comparison website uSwitch, 41% said they would not take advantage of the new employment laws, with some blaming the fear of damage to career prospects.
More than half of those who would not take the leave said that they simply could not afford to take more time off, or work pressures would make it impractical.
The new paternity leave rights that come into effect on 3 April will mean that new fathers can take up to six months, if their partner returns to work.
Ann Robinson, Director of Consumer Policy at uSwitch, said: "The government may have recognised that fathers today are a lot more hands-on than they may have been in the past, but putting legislation in to support this is only part of the battle. The biggest fight will be to change social opinion, fears and prejudices and this is not going to happen overnight."
From 3 April, if a mother has returned to work, her partner can take leave between 20 weeks and one year after their child is born or placed for adoption.