By Panu Wongcha-um

BANGKOK (Reuters) -Thailand's billionaire former Prime Minister Thaksin Shinawatra said on Tuesday he would return home in July after 17 years in self-imposed exile, just days ahead of an election his party is expected to win.

Although Thaksin, arguably Thailand's most influential and divisive former leader, has failed to make good on numerous pledges to return, his remarks on Twitter were the first time in recent years that he has set a timeframe.

"I am seeking permission again. I have decided to come home to see my grandchildren within July, before my birthday," he said.

"I want permission," he added, without saying from whom he was seeking it. "It has been 17 years that I have been separated from my family. I am old."

The 73-year-old former police colonel and telecoms mogul has loomed large over Thai politics since his ousting in a 2006 coup. He left into exile in 2008 to avoid jail for abuse of power, a charge he insists was politically motivated.

Pheu Thai, a party controlled by his family and business allies, has held a big lead in opinion polls ahead of the May 14 vote, as its predecessors did before winning every election since 2001.

Thaksin's youngest daughter Paetongtarn, 36, is a prime ministerial candidate for the party and gave birth to her second child last week.

The latest polls show parties in the ruling, military-backed coalition far adrift of Pheu Thai, including the United Thai Nation party of Prime Minister Prayuth Chan-ocha, a general who led a coup in 2014 against the government of Yingluck Shinawatra, Thaksin's sister.

Asked by reporters about Thaksin's potential homecoming, Prayuth said: "It is up to him, it is up to the justice system."

In a later Twitter comment, Thaksin said he would not "be a burden to Pheu Thai" and his return would follow legal processes. He did not elaborate.

Thaksin has spent most of his exile in Dubai or in London, where he became famous for buying Premier League football club Manchester City in 2007.

His early investments strengthened a struggling side that went on to win six league titles under new owners.

Prime minister from 2001 until his overthrow in 2006, Thaksin built a political empire by courting millions of disenfranchised working class voters with populist policies.

But his family and business clique is bitterly opposed by some of Thailand's most powerful families and institutions, including the military.

(Writing by Martin Petty; Editing by Clarence Fernandez and Lincoln Feast)