Labour's Douglas Alexander claimed it was better for all sides to take part in the new round of talks in Switzerland.
The MP for Paisley and Renfrewshire South said: "It will be a matter of regret if Iran is not at these talks, because on any reckoning, given its heavy responsibility supporting Assad - providing troops, providing weapons - it is a key actor on the Syrian stage.
"Is it better for there to be talks, albeit with genuine and continuing disagreements between the parties, even about what the talks are aiming for, or to have no talks at all?
"The only basis to get that inclusive political settlement is to have talks, often with parties who disagree profoundly, indeed who are trying to kill each other within Syria, which is why what happens on Wednesday is I hope the start of another chapter in the Syrian story."
Yesterday Syrian state media dismissed a report by Russian news agency Interfax that President Assad had told visiting Russian parliamentarians he has no intention of giving up power.
Syria's main political opposition group in exile agreed on Saturday to attend the talks, dubbed Geneva 2, and said that three rebel groups supported the move.
But the fractured National Coalition itself has little influence on the ground in Syria and other major opposition fighter units have rejected its authority and peace talks.
The United Nations hopes the talks will bring about a political transition in the country, and US Secretary of State John Kerry said last week that Syria's future had no place for Mr Assad.
Syria, however, said in a letter to UN Secretary-General Ban Ki-moon last week that its focus at the conference would be on fighting "terrorism".
Mr Assad reportedly insisted again yesterday he is not prepared to step down despite rebel forces seeking a transitional government.