Artur Mas wants Catalonia to hold a Scottish-style vote on 9 November, but does not have the backing of the central government in Madrid.
Spanish PM Mariano Rajoy has said he will block any referendum.
Catalonia, which includes Barcelona, is one of Spain's richest and most highly industrialised regions, and also one of the most independent-minded.
On 19 September Catalonian lawmakers voted by a margin of 106 to 28 in favour of authorising the referendum, known locally as a "consultation".
Mr Rajoy and the Spanish government believe any vote would be illegal.
The prime minister is expected to take action at a special cabinet meeting early next week, and is likely to take the dispute to the country's Constitutional Court.
However, Mr Mas says he can use local laws to hold a vote in a matter of weeks.
The BBC's Tom Burridge in Madrid says the focus of attention now will be on how far the Spanish government is prepared to go in order to stop a referendum.
The Spanish government has said a referendum on Catalan independence would be unconstitutional, despite protests
Mr Mas has previously insisted that the pro-independence movement would prevail, even if it faces stiff opposition.
"If they think in Madrid that by using legal frameworks they can stop the will of the Catalan people, they are wrong," he said in the wake of the Scottish "No" vote.
Until recently, few Catalans had wanted full independence, but Spain's painful economic crisis has seen a surge in support for separation, correspondents say.
There is resentment over the proportion of Catalan taxes used to support poorer regions.
The pro-independence movement in Catalonia believes that the region can go ahead with the independence vote after the decree is signed.
Earlier this month hundreds of thousands of Catalans formed a "V" for "vote" along two of Barcelona's main roads calling for their right to vote.