The 195 members of the 290-strong parliament who signed the letter in part blamed satellite television for feeding the trend, ISNA news agency reported today.
A defining feature of Iran's interpretation of Islamic law since the 1979 revolution, hijab obliges women to cover their hair and much of their body in loose clothing when outside, regardless of their religion.
A dedicated "morality police" has long handed out fines, verbal notices or even arrested women it considers are not properly observing the rules, but lawmakers have in recent months criticised lax enforcement.
Iran's parliament is dominated by conservative males.
However, President Hassan Rouhani, a self-declared moderate who was surprisingly elected last June, has expressed a desire to expand social freedoms -- to the disapproval of hardliners.
In October, he asked police to be moderate when enforcing the hijab requirements and recently said, "We cannot take people to heaven by using whips," a remark that was condemned by conservatives.
Police in Tehran earlier this month launched a new drive against non-compliance of the female dress code. Officers were deployed on the capital's biggest roads, and women -- drivers and passengers -- checked.
The MPs letter and push for stronger enforcement coincides with an online campaign in which hundreds of Iranian women posted pictures of themselves flouting the dress code inside Iran.
