BANGKOK, Feb 20 (Reuters) - Thai Prime Minister Prayuth Chan-ocha has gained ground on frontrunner Paetongtarn Shinawatra in a new survey on Thailand's top choice for premier, which showed the former coup leader was the most popular choice in the capital Bangkok.

Political newcomer Paetongtarn, the daughter of ousted former Prime Minister Thaksin Shinawatra, was backed by 28.5% of the 1,571 respondents in the Feb. 17-18 survey by Super Poll released on Monday, closely followed by Prayuth on 25.7%.

With an election due by May, the survey will be a boost for former army chief Prayuth, 68, who garnered less than half of the support of Paetongtarn, 36, in polls since September by the National Institute of Development Administration.

The survey by Super Poll, a private research agency, showed Prayuth as the top choice in Bangkok among a third of those surveyed, while Paetongtarn was picked by 21.3% of respondents in the capital.

Paetongtarn was the choice of 29.9% of Thais outside of Bangkok, compared with Prayuth on 24.3%.

The election will showcase the bitter rivalries at the heart of 17 years of on-and-off political tumult in Southeast Asia's second-biggest economy.

Paetongtarn's Pheu Thai party and its previous incarnations have won every election in the past two decades, drawing their main electoral strength from the populous, largely rural northeast of the country. But three of their governments have been removed from office, by military coups and court rulings.

Prayuth, who has switched to a new party, led the coup against the administration of Yingluck Shinawatra - Paetongtarn's aunt - in 2014 and has been in power ever since.

Paetongtarn at the weekend told Reuters military intervention had made Thailand go backwards and said it was time for bigger reforms to address festering problems.