BANGKOK, Feb 7 (Reuters) - Prime Minister Thaksin Shinawatra was set on Monday to head Thailand's first single-party government following a big election win that was widely expected but was turning out to be more one-sided than forecast.

An exit poll conducted by six television channels and a Bangkok university, unveiled minutes after Sunday's voting ended, said Thaksin's Thai Rak Thai party would win 399 of 500 seats in parliament.

If borne out by the official tally, it would be the first time in Thailand's coup-prone history that an elected leader has won a second term.

Final results, delayed by complaints of fraud and cheating, were expected later on Monday. But Thaksin has already claimed victory and the opposition has conceded defeat.

Financial markets were happy with the presumed victory for the former telecommunications tycoon and the stock market climbed to its highest in a year in early trade.

The markets expect the size of his mandate would enable Thaksin to push through investor-friendly measures such as partial privatisations of state utilities and reform of the telecoms sector.

Credit agency Standard & Poors said Thaksin could improve Thailand's growth prospects with structural reform of the economy, but said the size of his win might undermine checks and balances on government.

"If these fears are realised and it resulted in increased corruption and a marked rise in populist strategies, Thailand's credit-worthiness could suffer," Singapore-based analyst Philippe Sachs said.

Thaksin, 55, promised Thailand's 63 million people strong and purposeful leadership in his second term.

"It will be much better because we have strategies, we have plans and we will push them forward," he said.

The first elected Thai leader to complete a full term, he promised to eradicate poverty, spend $60 billion on new infrastructure and push ahead with privatising state firms despite fierce opposition from unions.

"SINGLE PARTY RULE"

By 0400 GMT, the vote count suggested Thaksin's party, whose name means "Thais Love Thais" in English, had won 308 out of 395 seats tallied and pointed to an ultimate 375, a mammoth mandate in a country used to coalition government.

When Thaksin first won in 2001, his party clinched 248 seats.

"Single party rule", the Bangkok Post newspaper declared in a front page headline on Monday, ramming home the significance of the victory in an election the opposition billed as a referendum on what it called one-party dictatorship.

Even though the result looked likely to beat pre-election estimates, Thaksin appeared to take it all in his stride.

"It's not really a big surprise," he told reporters at party headquarters after a lacklustre campaign some analysts said was largely a referendum on his CEO-style of leadership.

The main opposition Democrat Party conceded defeat to Thaksin, one of Thailand's richest men who swept to power in 2001 on a platform of cheap health care and rural handouts.

"I would like to congratulate Thai Rak Thai for achieving its goal of being a single-party government," said Democrat leader Banyat Bantadtan.

The Democrats looked set to win 80 seats, far fewer than the 201 they had appealed for, leaving Thaksin's party set to establish sole grip on power.

The all-pervading influence of Thaksin has been unaffected by unrest in the Muslim south and buoyed by his speedy response to the Dec. 26 tsunami.

He is accused by critics of being authoritarian and they say the staggering election win will only strengthen that tendency. However, Thaksin himself dismissed the suggestion.