He was told the weapons were there in two reports written by some of the UK's top spies and secret agents.
But a row has started over one of the reports.
Bits of it were apparently written 12 years ago by a US university student. Some people aren't happy about that because they think the info was dodgy.
That's made them question other things in the report. But Mr Blair says all the other bits are true and that using UK soldiers in Iraq was okay.
Some soldiers from the UK and the US are still in Iraq trying to make the country safer, but it's still dangerous for them to be there.