ATHENS (Reuters) - Greece's conservative New Democracy party said on Monday that a small splinter party led by former foreign minister Dora Bakoyannis had agreed to rejoin it for the June 17 election, crucial for the country's future in the euro zone.

Political deadlock after the inconclusive May 6 election and the inability of warring parties to forge a coalition government have forced Greece to hold a second election as the threat of economic collapse grows.

Bakoyannis's small liberal Democratic Alliance party won 2.55 percent of the vote on May 6, less than the 3 percent required to enter parliament.

The pro-bailout New Democracy, punished by voters tired of austerity, came first with 18.9 percent and the leftist SYRIZA, riding a wave of discontent and promising to end the austere policies that are deepening Greeks' economic hardships, came second with 16.8 percent.

European leaders say that if the next Greek government spurns the bailout, they will have no choice but to cut off funding, which could force the country out of the euro.

The prospect sent the single currency tumbling last week and hurt the bonds of Spain and Italy, countries that could be next in the firing line if Greece collapses.

"I welcome the decision of Ms. Bakoyannis to suspend the operation of her party and take part in our common effort," New Democracy leader Antonis Samaras said.

"We have called for a big patriotic front to unite all citizens who will not let the country surrender to populism and be led to bankruptcy, international isolation and be forced to exit Europe," he said.

The latest opinion polls show that SYRIZA has edged ahead of New Democracy, which has also regained some of its lost support.

SYRIZA is ahead with 25.1 percent, and New Democracy a close second with 23.8 percent, one poll showed.

"We will defend our country's future and its European perspective. Our goal is to get the country out of the crisis but keep it inside Europe and the euro," Samaras said.

(Reporting by George Georgiopoulos; Editing by Tim Pearce)