Blair, who won a historic three elections in a row before handing power to Brown in 2007, has played a relatively low key role so far in the campaign.
Although he has the charisma that many feel Brown lacks, Blair remains a divisive figure among Labour supporters and the wider British public.
Asked if Brown had failed to get his message across in three live televised election debates, Blair said: "I don't think he has failed at all.
"If people actually look and listen to the substance, they will see someone who is completely on top of his facts."
The closest British election race in nearly 20 years is heading for an inconclusive result, with polls suggesting the main opposition Conservatives are in the lead, but uncertain of securing a majority in parliament.
Allowing Blair to join the campaign trail is considered risky by many political analysts.
Supporters say he is one of Labour's best leaders, but his strong electoral popularity early on slumped when he backed the US-led invasion of Iraq in 2003.
A million people marched in London against the war and Blair was later accused of exaggerating the evidence that underpinned the case for war.
Unlike Blair and David Cameron, the Conservative leader, Brown is seen as a poor media performer.
Cameron came top in snap polls following Thursday's final televised leaders' debate, with Nick Clegg, the leader of the Liberal Democrats, second and Brown trailing in third.
The three men clashed on a range of economic issues, including taxes, the banking sector and the decline of British manufacturing industry.
They were also quizzed on immigration, a topic which has surfaced in each of the three debates and triggered Brown's "bigot" outburst on Wednesday.
Cameron said on Friday that the election was "far from won" while Clegg said the poll was now a "two-horse race" between his party and the Conservatives.
In a further blow to Brown's chances, Britain's Guardian newspaper, long considered a Labour stalwart, said it would be backing Clegg for next week's poll.