(Adds Treasury secretary, finance minister comments)

By Marcela Ayres

BRASILIA, Oct 27 (Reuters) - Brazil's central government budget deficit widened more than expected in September as a deepening recession hurt tax revenues, the finance ministry said on Thursday.

The central government's primary deficit, which includes federal ministries, social security and the central bank, rose to 25.303 billion reais ($8.027 billion) in September from 20.345 billion reais in August.

The record gap for a month of September highlights President Michel Temer's uphill battle to mend Brazilian public finances after years of hefty spending. Analysts have said it will probably take years for Latin America's largest economy to earn back the investment-grade credit rating it lost in 2015.

The government will stick to its 2016 budget target, Treasury Secretary Ana Paula Vescovi told journalists, counting on extraordinary tax revenues to be paid as part of an amnesty program for the repatriation of assets held abroad.

Brazil's central government primary deficit so far this year rose to 96.633 billion reais, the finance ministry said. Earlier on Thursday, Finance Minister Henrique Meirelles said extraordinary revenues under the asset repatriation program were likely to surpass 50 billion reais.

The central bank will release on Monday the September public sector primary deficit, which includes states and municipalities. Economists in a monthly poll by the Finance Ministry have forecast a 2016 deficit of 159.9 billion reais, slightly better than an official target of 163.9 billion reais.

The central government had been expected to post a deficit of 23.780 billion reais in September, according to the median forecast in a Reuters poll of 11 economists.

Finance ministry data earlier on Thursday showed federal tax revenues fell 8.27 percent from September 2015, also missing market expectations.

"Slumping economic activity explains a lot of that. Companies are also facing a lot of cash flow and credit problems, making them delay tax payments," economists with local consultancy Rosenberg Associados wrote in a note to clients.

Brazil's severest recession in at least eight decades has taken a turn for the worse in recent months, frustrating economists who expected it to stabilize following the impeachment of unpopular President Dilma Rousseff.

Business and consumer confidence have lifted off record lows but measures of economic activity including industrial output, retail sales and services growth have fallen sharply, suggesting a mild recovery will only start in early 2017.

($1 = 3.1521 Brazilian reais) (Additional reporting by Lisandra Paraguassu; Writing by Silvio Cascione; Editing by James Dalgleish and Tom Brown)