An Israeli court has extended the prison sentence of the former prime minister Ehud Olmert by a month after rejecting a plea-bargain over an obstruction of justice charge.

Olmert, 70, will begin serving his now 19-month term on Monday, making him the first former head of government in Israel to go to prison.

Olmert, prime minister from 2006 to 2009, was convicted of bribery charges related to his 1992-2003 term as Jerusalem’s mayor and real estate deals in the city.

He was also accused separately of obstruction of justice over allegations he tried to persuade a former aide not to testify against him.

Olmert reached a plea bargain on that charge, agreeing to a six-month sentence that would be served concurrently with his 18-month term.

But a Jerusalem district court on Wednesday partly rejected the deal, ordering him to serve five months concurrently and one independently, and his lawyers said they would consider an appeal.

A centrist, Olmert was credited with working towards a peace settlement with the Palestinians until the graft scandal forced him to step down.

Olmert was found guilty in 2014 of two bribery charges – accepting 500,000 shekels (£90,000) from developers of a Jerusalem real estate project and 60,000 shekels in a separate land deal. He was sentenced to six years in jail by a judge, who said Olmert’s crimes as a public official were akin to treason.

Ruling in December on his appeal, the supreme court said it had not been proven beyond a reasonable doubt that Olmert had solicited the 500,000 shekels, and cut his jail term to 18 months.

A separate eight-month prison term is pending since Olmert’s conviction last May in another case related to cash payments he was alleged to have received from an American businessman. His appeal against that verdict is being considered by the supreme court.