He called on fellow "Remain" supporters to "mobilise and to organise" against proponents of Brexit, writing in The New European newspaper that "we're the insurgents now".
"There is absolutely no reason why we should close off any options," he later told BBC radio.
"We are entitled to carry on scrutinising, and, yes, if necessary, to change our minds. This is not about an elite overruling the people.
Blair, who was prime minister from 1997 to 2007, called Brexit as "catastrophe", and revealed that he had recently held talks with French President Francois Hollande that had highlighted the challenges Britain faces in upcoming negotiations to set the terms of the break-up.
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"We are not going to be conducting this negotiation with a group of European businessmen who may well decide that what they want is the maximum access into the UK, and they may be prepared to be quite forgiving.
"The people we will be conducting this negotiation with will be the political leaders of the European Union, and their parliaments, so this is going to be a negotiation, in my view, of enormous complexity."
He said that Britain would either have to accept obligations in return for access to the single market or face "severe" economic consequences.
"Instead of desperately trying to find ways to thwart the will of voters and talking down Britain's prospects, Labour should be concentrating on helping to make a success of Brexit," she added.