An Israeli court has extended former Prime Minister Ehud Olmert's 18-month jail sentence for bribery by a month, BBC News reports.

The court rejected a plea bargain that would have seen Olmert serve six months for obstruction of justice concurrently with the bribery sentence. Instead, he will serve five months concurrently and one consecutively.

Olmert, Prime Minister from 2006 to 2009, will become the first former head of government in Israel to go to prison. The 70 year old was handed a six-year sentence in 2014 after being found guilty of accepting, while he was mayor of Jerusalem, a 500,000-shekel ($129,000) bribe from developers of one real estate project and a 60,000-shekel bribe in connection with another project.

But the Supreme Court subsequently cleared him of the charge of accepting the 500,000-shekel bribe and reduced his sentence to 18 months.

The Supreme Court is yet to rule on an appeal by Olmert against an eight-month prison sentence he was handed last year after being convicted of fraud and breach of trust for accepting illegal payments from an American businessman.

Olmert has denied accepting bribes or illegal payments, but did admit to attempting to persuade his former secretary not to testify against him.