Mr Blair cautioned that the Prime Minister is making decisions which will “determine the future of the country for generations to come”, but said he acknowledged the “very tough position” she is in.
“I have got lots of sympathy for her personally and I think it’s a very, very tough position to be in.
“I can tell frankly that she’s obviously very tired, probably exhausted, with it all and it’s a huge strain,” he told BBC Radio 4’s Week in Westminster.
“All of that is obviously true but unfortunately the decisions that are being taken are going to determine the future of the country for generations to come.”
Mr Blair, a strong proponent of a second referendum, also called for a proposed series of “indicative votes” on alternatives to Mrs May’s Brexit deal to be delayed, saying it would be better to have a debate in a less frenzied atmosphere.
He said: “The most difficult thing for MPs will be: do they try and do these so-called indicative votes fast and get a conclusion by 12th April?
“I personally would not do that, but I think they’ll be under huge pressure to do it. It would be better if we took a longer time to do it.”
He added: “Really what I’m saying is because we spent so long not really grasping what a soft Brexit means, the so-called Norway option, what a hard Brexit means, the so-called Canada option, it would be better if you had a debate that happened in less of an atmosphere of frenzy.
“But obviously the downside of that is that you then have to have a longer extension and then the question of the European elections and so on comes up so I understand all the reasons against it but I think the most important thing to realise is that at some point Parliament is going to have to nail its colours to the mast.”