Finland's 2000 world champion Marcus Gronholm won the British rally for the first time in a Peugeot.
Gronholm was two minutes and 27.1 seconds ahead of compatriot and team-mate Harri Rovanpera.
Reading-born Burns will officially become champion when he makes the 20-mile
trip from the final stage at Margam to the formal finish in front of
Cardiff's City Hall.
He triumphed despite winning just one of the 14 rounds - in New
Zealand.
A consistent late-season run brought 29
points in the last six events.
Burns said: "It is a shame we could not win the rally, but after the
first day all I was thinking about was the championship.
Burns shelters from the rain between stages
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"We have been so close for the past two years and we had
a terrible first half of the season, but the second half
has been just fantastic.
"There was a massive amount of
pressure on myself and all of us before this rally
"It
becomes high risk so it is a relief to come through."
The Andorra-based driver had been championship runner-up for the past two
years.
On Sunday, he safely negotiated the four remaining special stages, totalling 67 miles,
in the muddy south Wales forests.
The third-place finish gave Burns 44 points, two ahead of Colin
McRae and three over Tommi Makinen.
McRae, the 1995 champion, and four-time champion Makinen crashed out of the race on Friday, leaving Burns with a clear shot at the title.