Speaking on the BBC's Newsnight programme last night, Barroso dismissed the idea that the "emergency brake" on in-work benefit payments in Cameron's renegotiation would work.
The emergency brake would temporarily allow the UK to not make welfare payments to new EU migrants.
Here is what Barroso said.
BBC Do you think it would put anyone off coming here to work?
Barroso No. Frankly not.
It's very interesting that Barroso was so open in saying that the emergency brake wouldn't work, because for a long time he has been one of the biggest advocates of the EU and is a big supporter of the UK remaining in it.
By saying that the most high profile bit of Cameron's renegotiation won't do what it is intended to do, Barroso is basically saying that Cameron is wrong to be using it to try and convince the British public to remain in the EU.
The emergency brake was the hardest thing for Cameron to get EU leaders to consider approving in his proposed deal to renegotiate Britain's membership of the EU and he has told the British public that it will be a big deal if EU leaders approve it next week.
The whole idea is that it will discourage some EU immigrants from coming to the UK and migration is one of the main reasons that is making people consider voting to leave the EU.
If Cameron does convince EU leaders to sign up to the deal, he will call a referendum on Britain leaving the EU and will use the renegotiation deal to campaign for Britain to stay in the union.
You can watch the video of Barroso talking to Newsnight below.