"European leaders, after discussions that I have had, are willing to consider changes to go in the direction of the United Kingdom, including the freedom of movement".
Blair served as British prime minister for 10 years from 1997, winning a landslide victory that kept the Labour government in power for years.
Former British prime minister Tony Blair speaks to media reporters after making a presentation on countering violent extremism through education during an open sessin of the United Nations Security Council Committee on Counter-Terrorism, at the UN headquarters in NY, on November 21, 2013.
Pro-EU voters will eventually begin to understand Labour holds the "same damaging position" as the Tories, to leaving the single market to end free movement, and the party should instead champion a "radically distinct" position on Europe.
He also took a few jabs at his Labour successor, Jeremy Corbyn by saying he won't be able to deliver on a "jobs first" pledge. "I think they thought that the likelihood was that the Tories would be the government, but were determined to neuter the mandate".
Blair said political changes on the continent, including the election of Emmanuel Macron in France, have made an alternative to a hard Brexit more likely.
"Rational consideration of the options would sensibly include the option of negotiating for Britain to stay within a Europe itself prepared to reform and meet us halfway", Blair, who was prime minister from 1997 to 2007, wrote.
The former Prime Minister claims he has held talks with leaders about how the United Kingdom could stay "within a reformed Europe".
"Yet this option is excluded".
He said such a scenario might include a compromise on freedom of movement - a key European Union principle that conflicts with Britain's goal of placing limits on immigration.
German and French voters would favour similar controls, the polling suggested.
"This is causing us real damage".
"Europe knows it will be poorer and less powerful without us". That is what we need now.
"On any basis, leaving is complex and will take years". But he called Brexit the biggest political decision since World War II.
Blair told Sky News Saturday: "I think it's absolutely necessary that it (Brexit) doesn't happen because I think every day is bringing us fresh evidence that it's doing us damage economically, certainly doing us damage politically".