County councillors agreed to the bid by UK Coal to extract 430,000 tonnes of coal from the site at Stobswood.
Work will go on until 2006 when the company has promised to restore the land back to a greenfield site.
Protestors from villages around the Stobswood site said they were angry and disappointed because the noise and dust from the works ruins their quality of life.
Conditions imposed
The mine opened more than a decade ago and about 12.2 million tonnes of coal has been extracted in that time.
Protesters fear the plans for further extractions could herald more applications in the future.
The mine is the largest in the country - 200m deep in parts - and employs about 300 people.
Northumberland County Council has imposed three conditions on their approval of UK Coal's plans.
'Fight continues'
Mary Campbell, environment officer, told BBC Look North: "The three critical conditions are that there should be no night-time working, secondly... coaling should cease in April 2005 and not April 2006.
"Thirdly, and the most important one, it would require additional measures put in place for the control of dust, including chemical dust suppressants."
But campaigner Alan Williams has vowed to continue his protest against the mine.
He said "UK Coal may have won for today but this is very much the start, not the end, for us."