Tony Blair has admitted he did not realise how many migrants would come to the UK when he opened Britain's borders to millions of European workers.
The former Labour leader relaxed immigration controls in 2004 after 10 new nations including Poland, Lithuania and Hungary, were admitted to the EU.
He tried to play down the significance of opening Britain's borders, arguing that most EU migrants came to the UK after 2008.
However, official figures show that the number of EU migrants who came to Britain rose from just 15,000 in 2003 to 87,000 the following year. That figure increased to 104,000 in 2006 and 127,000 in 2007.
Mr Blair also made a factual error by suggesting that he could only have imposed transitional controls, temporarily barring migrants for four years.
In fact other EU nations including Germany introduced the measures for up to seven years.
Asked on the BBC's Andrew Marr show if he knew how net migration would rise, Mr Blair said: "No we didn’t know the numbers.
"But by the way it's very important to realise two things. When these countries joined the EU, [it was] very important for us that they did join the EU - important for our security, important for our economy.
"There was freedom of movement of people immediately. We could have delayed for four years their ability to come here and work.
"We didn’t, it’s true, the economy was in a completely different position in 2004. But I just point out the majority of EU immigration came post-2008.
"One of the tragedies of Brexit is we're now in a situation where where we think this enlargement of the EU, like the single market, was some sort of error.
"It was actually a bi-partisan policy of both governments that has done great benefit to this country."
He made the comments as he launched his new 'Institute for Global Change', which intends to develop new center ground policy ideas.
Speaking on Sunday he said the British people still may change their minds over the “tragedy” of Brexit.
He said: “My view is very simply I agree at the moment the argument for many people is over. But if, as you go down this path, the British people realise three things that I think it could change."