Madrid (AFP) - Catalonia's president vowed Friday to begin seceding from Spain if pro-independence parties win a majority of seats -- not votes -- in regional parliament elections set for later this month.

Catalonia's main pro-independence parties have agreed for the first time to run on a single unified list dubbed "Together for the Yes" in the September 27 regional vote.

The goal is to turn the ballot into a referendum on independence and secure a popular mandate for a break with Spain.

"If the all the options which are in favour of the yes have an absolute majority of lawmakers we will move forward," Catalan president Artur Mas said during an interview with Onda Cero radio.

Under Catalonia's proportional representation system Mas's pro-independence coalition could win a majority of 68 seats in the 135-seat Catalan assembly even if it gets fewer votes than parties that oppose independence.

Unlike Scotland's referendum on independence, which was recognised by London and resulted in a vote to stay in the United Kingdom, Spain's conservative Prime Minister Mariano Rajoy has refused to allow a plebiscite in Catalonia on the grounds it would be against the constitution.

If the pro-independence coalition wins the elections, their goal is to push through an 18-month roadmap for secession for the region of some 7.5 million people which accounts for a fifth of Spain's economic output.

According to the most recent polls 50 percent of Catalonia's people are opposed to independence, while 43 support breaking from Spain.

Asked if he would push ahead with the secession plan even if a majority of Catalans oppose it, Mas said: "Yes, I have been forced to count the lawmakers."

"I prefer to count ballots, that is what I have tried for two and a half years," he added.

Mas staged a symbolic independence referendum in November 2014 after the government used the courts to block his plan to hold an official referendum.

About 80 percent of the 2.2 million people who voted backed secession, but the turnout was little more than 40 percent.