A Saudi woman has been sentenced to be lashed 10 times with a whip for defying the kingdom's prohibition on female drivers.

It is the first time a legal punishment has been handed down for a violation of the longtime ban in the ultraconservative Muslim nation.

Police usually stop female drivers, question them and let them go after they sign a pledge not to drive again. But dozens of women have continued to take to the roads since June in a campaign to break the taboo.

The sentence comes two days after King Abdullah promised to protect women's rights and decreed women would be allowed to participate in municipal elections in 2015. Abdullah also promised to appoint women to the all-male shura council advisory body.

The mixed signals highlight the challenge for Abdullah, known as a reformer, in pushing gently for change without antagonising the powerful clergy and a conservative segment of the population.

Abdullah said he had the backing of the official clerical council. But activists saw Tuesday's sentencing as a retaliation from the hardline Saudi religious establishment that controls the courts and oversees the intrusive religious police.

"Our king doesn't deserve that," said Sohila Zein el-Abydeen, a prominent female member of the governmental National Society for Human Rights. She burst into tears in a phone interview and said: "The verdict is shocking to me, but we were expecting this kind of reaction."

The driver, Shaima Jastaina, who is in her 30s, was found guilty of driving without permission, activist Samar Badawi said. The punishment is usually carried out within a month. It was not possible to reach Jastaina, but Badawi, in touch with Jastaina's family, said she had appealed against the verdict.

Saudi Arabia is the only country in the world that bans women – both Saudi and foreign – from driving. The prohibition forces families to hire live-in drivers, and those who cannot afford the $300 to $400 (£190 to £255) a month for a driver must rely on male relatives to drive them to work, school, shopping or the doctor.

There are no written laws that restrict women from driving. Rather, the ban is rooted in conservative traditions and religious views that hold giving freedom of movement to women would make them vulnerable to sins.

Activists say the religious justification is irrelevant.

"How come women get flogged for driving, while the maximum penalty for a traffic violation is a fine, not lashes?" Zein el-Abydeen said. "Even the prophet [Muhammad's] wives were riding camels and horses because these were the only means of transportation."

Since June, dozens of women have led a campaign to try to break the taboo and impose a new status quo. The campaign's founder, Manal al-Sherif, who posted a video of herself driving on Facebook, was detained for more than 10 days. She was released after signing a pledge not to drive or speak to media.

Since then, women have been appearing in the streets driving their cars once or twice a week. Until Tuesday, none had been sentenced by the courts. But recently, several women have been summoned for questioning by the prosecutor general and referred to trial.

Najalaa al-Harriri, a housewife, drove twice, not out of defiance, but out of need, she said. "I don't have a driver. I needed to drop my son off at school and pick up my daughter from work."

"The day the king gave his speech, I was sitting at the prosecutor's office and was asked why I needed to drive, how many times I drove and where," she said. She is to stand trial in a month.

After the king's announcement about voting rights for women, Saudi Arabia's Grand Mufti Abdel Aziz Al Sheik blessed the move and said: "It's for women's good."

Al-Harriri, who is one of the founders of a women's rights campaign called My Right My Dignity, said: "It is strange that I was questioned at a time the mufti himself blessed the king's move."

Asked if the sentencing would stop women from driving, Maha al-Qahtani, another female activist, said: "This is our right, whether they like it or not."