UPDATE 2-Brazil pension reform has one week to pass or slip to 2018

(Adds comments from Temer, Meirelles)

BRASILIA, Dec 12 (Reuters) - A proposal to overhaul Brazil's costly social security system will be put to a vote within the next week if it has enough support, President Michel Temer said on Tuesday, adding that otherwise it will have to be left for next year.

Speaking to business leaders, Temer warned that Brazil would have to make painful cuts in public spending like Portugal did if it puts off reform of a pension system that is unsustainable for public finances.

Temer has had to reduce the extent of the unpopular measure to get it through Congress, but said the current version would still save the Treasury 600 million reais ($180 million) a year, down from 800 million reais in an earlier plan.

Investors are watching closely for progress on the bill, which many consider crucial to reining in Brazil's surging public debt. 2018 is an election year and the unpopular bill may face more push back from lawmakers seeking re-election.

Brazil's currency, the real, slipped as much as 1 percent on Tuesday, flirting with its weakest levels in nearly half a year.

Finance Minister Henrique Meirelles called the overhaul essential to bringing a huge budget deficit under control.

At the same gathering, Meirelles said social security was consuming 50 percent of government spending and warned that it would rise to 80 percent in 10 years if nothing was done.

Earlier in Sao Paulo, Meirelles said he expected debate on the pension reform bill to start in the lower house of Congress on Thursday followed by a vote next week, the last before Christmas recess.

Speaker Rodrigo Maia warned that Temer's government was still short of the three-fifths super majority, or 308 votes, needed to pass the pension bill. He called it suicide to go ahead with a planned Dec. 18 vote without the votes in hand.

The latest survey of lawmakers by political consultancy Arko Advice shows that Temer still needs to secure some 35 votes.

The bill proposes increasing the age at which Brazilians can retire and collect social security. It would also make pension payouts in Brazil, among the most generous in the world, more modest, and particularly for public-sector employees.

The bill must be approved twice in each of both chambers. Its approval in the Senate is expected to be easier.

($1 = 3.31 reais) (Reporting by Ricardo Brito and Anthony Boadle; Additional reporting by Iuri Dantas in Sao Paulo; Editing by Richard Chang)