The ruling Justice and Development Party (AKP) wants to criminalise adultery as part of penal code amendments that are otherwise aimed mainly at bringing it into line with European norms.
Parliament meets in a special session on Tuesday to debate the penal code overhaul, and with its huge AKP majority it looks sure to pass the adultery clause unless the government decides to water it down or postpone it in the face of EU disapproval.
If it is passed a legal tussle will almost certainly ensue with President Ahmet Necdet Sezer, who has a record of vetoing legislation he regards as anti-secular.
A final draft of the adultery clause has not been published as the government says it is still under discussion, but AKP officials say it envisages jail sentences of up to two years.
The government says criminalising adultery will protect the family and strengthen women's rights - a major EU concern - by giving them legal clout against unfaithful husbands.
Fears in some quarters that the law would be a "snooper's charter" appear to be eased by reports that only a husband or wife could initiate proceedings rather than third parties such as the state, as is the case in some Muslim states.
However, the idea of jailing people for conduct that in Europe would at most be handled under civil law has outraged Turkish liberals and women's groups and sparked EU warnings that it could harm Ankara's chances of a start date for accession talks.
"This has come from Erdogan himself. There are a number of deputies in the AKP who think this is not a proper idea, or if it is a proper idea it's not the proper time... but Erdogan is taking it as a challenge to his power"
Belgian Foreign Minister Karel De Gucht was the latest to blast the plan on Sunday, echoing warnings by EU Enlargement Commissioner Guenter Verheugen that it gives the impression that Muslim but firmly secular Turkey is moving towards Islamic law.
The EU's verdict on Turkey is imminent - EU leaders will decide in December whether to open talks, drawing on a progress report next month by the executive European Commission - and many analysts are perplexed by the timing of the proposal.
"Either the AKP has no understanding of the concept of 'timing' or it is we who don't understand what is going on," commentator Mehmet Ali Birand wrote in a weekend media column.
Like other Muslim-oriented reforms mooted by the government, such as easing laws against wearing headscarves in public life, the adultery proposal has touched a raw nerve among liberal Turks who fear the AKP has a hidden, Islamist agenda.
The government, which sprang from a banned Islamist party but now rejects that label, quietly allowed the headscarf and similar controversies to fade off the immediate agenda after they raised concerns in Turkey's powerful secular establishment.
Some say it may do the same on adultery, perhaps feeling it miscalculated the EU's response.
Officials say the law will protect
the family and womens' rights
But others say Prime Minister Tayyip Erdogan, himself a devout Muslim, looks unlikely to back down this time.
"This has come from Erdogan himself," said Murat Yetkin of the liberal daily Radikal.
"There are a number of deputies in the AKP who think this is not a proper idea, or if it is a proper idea it's not the proper time ... but Erdogan is taking it as a challenge to his power."