Thailand's toppled former Prime Minister Yingluck Shinawatra has been banned from travelling overseas because graft charges carrying up to a decade in jail will be formally laid against her later this month, officials said Monday.

Yingluck, who was ousted by a court ruling shortly before the army grabbed power in May, has already been banned from politics for five years following her impeachment over a costly rice subsidy scheme.

In addition, she faces jail if found guilty of corruption charges linked to the policy, which paid farmers in her party's rural heartland well over the market rates for their crop.

Local media said Yingluck had wanted to travel to Hong Kong, China and Britain this week.

But the Thai junta denied her permission on the advice of the state prosecutor, the Office of the Attorney General.

"The office (OAG) said the case was still under the justice process and asked for the trip to be postponed," deputy prime minister General Prawit Wongsuwon told reporters.

"The ban was linked to the legal process."

An OAG spokesman said it would formally indict Yingluck by February 21, leaving the Supreme Court to decide if the case would proceed.

As Thailand's highest court, there is no right of appeal against a Supreme Court decision.

Yingluck's lawyer said there was no legal justification for blocking her from leaving the country.

"The case is yet to go to court," Norawit Larlaeng told AFP, adding a ban before any proceedings treated his client like she "was guilty without having a court ruling".

The generals seized power ostensibly to restore order after months of often deadly protests against Yingluck's government.

The demonstrators -- and their supporters among the Bangkok elite, judiciary and army -- accuse Yingluck of overseeing a corrupt and venal administration on behalf of her billionaire brother Thaksin, also a former premier.

Thaksin, loathed by the Bangkok establishment but loved by the northern rural poor, lives in self-exile to avoid jail for a corruption charge.

Shinawatra supporters accuse the Thai elite of using the courts to persecute Yingluck. She remains popular among her rural base, who have voted for the family's parties in every election since 2001.

Yingluck, who has largely remained quiet since the coup, decried her impeachment last month as the "death of democracy". A senior US envoy angered the junta by saying the legal moves against her could appear to be politically motivated.

Shinawatra governments have been knocked out by two coups and dizzied by the removal of three premiers by the interventionist courts since 2006.

The junta says it will hold fresh elections in early 2016 once reforms to tackle corruption and curb the power of political parties are codified in a new constitution.