The UK population surged by almost 500,000 in 2014 to hit 64.5 million as net migration pushed numbers up, official figures have revealed.
The Office for National Statistics (ONS) said the amount of people residing in Britain increased by 491,100 in the year to mid-2014, an increase above the average when compared with previous years.
The research body explained that net international migration of 259,700 over the period meant the immigration was the biggest contributor to the increase.
The data also showed there was a natural growth, when the number of births are deducted from the number of deaths, of 226,200 people.
The ONS said England (+450,800) accounted for the largest population growth as Wales (+9,600) recorded the lowest rise among the UK regions.
The figures will put further pressure on David Cameron as he attempts to reduce immigration levels towards his "hundreds of thousands" election pledge.
Earlier in June, the prime minister announced the government plans to launch a Migration Advisory Committee.
At the time, Cameron, speaking at Prime Minister's Questions (PMQs), said: "This government is on the side of working people: in the past, it has been too easy for businesses to recruit from overseas, undermining those who want to work hard and do the right thing.
"As part of our one-nation approach, pushed forward by my Immigration Taskforce, we have asked the Migration Advisory Committee to advise on what more can be done to reduce levels of work migration from outside the EU."
The body will attempt to reduce the number of non-European Economic Area (EEA) migrants moving to the UK.
The announcement comes after the ONS said net migration hit more than 318,000 in 2014.
The prime minister also wants to reduce EU migration by making substantial welfare reforms, including stopping the citizens from receiving benefits until they have lived in the UK for four years.
But the move requires the approval of Brussels as Cameron attempts to renegotiate with the EU ahead of a promised referendum on the UK's membership of the 28 member bloc by the end of 2017.
Meanwhile, a crisis has broken out in the French town of Calais as migrants attempt to illegally cross the English Channel by boarding lorries bound for the UK.