"We will be a minority in parliament," said Ali Akbar Mohtashami-Pur, spokesman for the "Coalition for Iran" - a grouping of eight reformist parties.
A powerful conservative-run political watchdog, the Guardians Council, has barred thousands of candidates from standing in the elections. Most of those disqualified are reformists, among them some of the movement's most prominent figures.
The main reformist party, the Islamic Iran Participation Front (IIPF) led by the brother of reformist President Muhammad Khatami, has decided to boycott the elections.
In such circumstances, Mohtashami-Pur said it was inevitable hardliners and conservatives would sweep to victory and admitted the reformist camp had set its sights low.
"This is not a fair competition," he told a press conference. "There is such a disproportion, and the conservatives have mobilised far more means than us - so even if we have just one seat, we have a victory and we have foiled the plans of the conservatives."
He said his bloc would only be contesting 218 out of the 290 seats up for grabs and admitted a list of candidates had not been printed yet to be distributed among voters.