Iranian MPs on Monday condemned a planned union between Saudi Arabia and fellow Sunni-ruled Bahrain, news agencies reported.
"Bahraini and Saudi rulers must understand that this unwise decision will only strengthen the Bahraini people's resolve against the forces of occupation," they said in a letter, referring to Saudi military support for Manama.
The letter, read out in the 290-member parliament and signed by 190 MPs, warned that "the crisis in Bahrain will be transferred to Saudi Arabia and will push the region towards insecurity."
Gulf Cooperation Council leaders at a summit on Monday were expected to discuss a Saudi proposal to develop their six-nation council into a union, reportedly starting with Saudi Arabia and Bahrain.
The exact nature of the union, first floated by Saudi King Abdullah in December, remains unclear. But Bahrain's state minister for information, Samira Rajab, has said it could follow the "European Union model."
The proposal comes after Bahrain was shaken last year by demonstrations led by its majority Shiite population calling for political reforms.
Shiite-dominated Iran supports the demonstrators' demands and strongly condemned a deployment of Saudi-led forces in Bahrain in March 2011 when a month of street protests was crushed.
Sheikh Ali Salman, the leader of Bahrain's main Shiite opposition formation Al-Wefaq, has said any union project must first be put to a referendum.
"The people of Bahrain alone have the right to" decide, he said, adding the kingdom's ruling "Al-Khalifa (dynasty) has no right to decide a union or confederation with any country."
The GCC was formed in 1981 when the Sunni-dominated monarchies of the Gulf aimed to bolster security after the 1979 Islamic revolution in Iran which was followed by an eight-year war between Baghdad and Tehran.