Thailand's military-backed government has summoned Singapore's ambassador to protest over ousted PM Thaksin Shinawatra's visit to the city state.
Mr Thaksin continues to make waves in Thailand
Thailand's leaders, who have been trying to sideline Mr Thaksin, were angry he was able to meet Singapore's Deputy Prime Minister S Jayakumar.
They are also thought to be unhappy at media interviews Mr Thaksin gave in Singapore, one of which was critical.
Mr Thaksin's government was ousted in a military coup in September.
The military-backed government stepped up its campaign against Mr Thaksin's regime on Tuesday as the former ruling party, Thai Rak Thai, went on trial accused of electoral fraud.
'Enough is enough'
Since being ousted from office, Mr Thaksin has travelled around Asia. The coup leaders fear these visits are politically motivated, and last week cancelled his diplomatic passport.
The Thai foreign ministry said it summoned Singapore's ambassador to discuss the meeting between Mr Thaksin and Singapore's Deputy Prime Minister, S Jayakumar, at the weekend.
"We informed the Singaporean (ambassador) that we are concerned by the political movements made by Thaksin," Thai Prime Minister Surayud Chulanont told reporters.
Singapore said the meeting was purely social and private.
But Thai officials rejected the explanation, withdrawing an invitation to Singapore's foreign minister to visit Thailand later this month.
In another sign that they are still worried by the former leader, the coup leaders asked broadcasters last week to limit coverage of comments by Mr Thaksin and his representatives.
A CNN interview with Mr Thaksin, conducted in Singapore and broadcast elsewhere on Monday night, was not shown in Thailand.
During the interview, Mr Thaksin said he had no wish to return to politics.
"Enough is enough," he said, adding that it was time to "contribute to the Thai society outside [the] political arena."
Mr Thaksin also spoke to the Wall Street Journal, saying that the military regime that had replaced his government should not be in power for long.
"We have tasted democracy, and we believe in democracy. So it's time for Thailand to restore democracy as soon as possible," he said.
He criticised the government's economic policies - such as sudden capital control measures which saw the stock market plummet, only to be partially rescinded the next day.
"Whether you like it or not, we have to live under a capitalist system," Mr Thaksin told the newspaper.
"To live in it successfully, we have to open up our economy and our society... Competition is unavoidable, so we have to prepare for it."
He also denied any involvement in deadly New Year's Eve blasts in Bangkok which killed three people.
"I'm not that kind of person," he said. "It's not my style."
Party on trial
Mr Thaksin's Thai Rak Thai party faces charges of illegally financing minor parties and misusing the election commission during controversial polls in April 2006.
The polls were subsequently annulled.
The main opposition to Thai Rak Thai, the Democrat Party, also faces trial later this week for obstructing campaigning during the same elections.
Both parties deny the charges against them, but face possible dissolution if found guilty. The verdicts for both trials are expected early in May.