Wilson Chowdhry from the British Pakistani Christian Association told Premier's News Hour the quota system in Pakistan, which gives every region of the country representation in institutions according to their population, isn't currently reflecting the number of Christians in the country.
Chowdhry said: "We believe that the current percentage of Christians shown on the population statistics of Pakistani of 1.6 per cent is in error.
"We believe the figure should be closer to between three to four per cent meaning that there should be between seven to eight million Christians in Pakistan."
The sixth most populous country in the world last held a census in 1998, despite a constitutional requirement for one to be held every decade.
The census is being carried out under tight security following three aborted attempts, with local reports that the potential political implications caused the delay.
However, Chowdhry said the census may not bring result in an accurate number of Christians being recorded.
He added: "It's that fear that has prevented Christians and other minorities from placing their Christian status on the election sheet when the questioners come round to access their faith.
"They feel that it will then be an opportunity for the government and other agencies - especially those that are extremist - to target them."
Pakistan is overwhelmingly Muslim but after Hindus, Christians are the second-largest minority group with estimates ranging from two to 10 million for Christians, and 2.5 to 4.5 million for Hindus.
Listen to Wilson Chowdhry speaking with Premier's Alex Williams:
