AFTER a three-year engagement, Jessica Simpson is so tired of waiting for wedding bells that she’s de­manded fiancé Eric Johnson tie the knot or take a hike.

The sexy singer’s heated ultimatum was triggered by her recent walk down the aisle as a bridesmaid for her close friend and publicist Lauren Auslander, says a source.

“After Jessica took on the role of glowing bridesmaid, she sat Eric down and told him, ‘Stop drag­ging your feet. Marry me soon or else!’” revealed an insider.

Jessica, 33, and her 34-year-old beau were all smiles at Aus­lander’s Oct. 11 fairytale wedding by the sea in Newport, R.I., but their blissful weekend took an ugly turn when the “Dukes of Hazzard” beauty blasted her fiance for refusing to commit to a wedding date.

“Jessica got caught up in the spirit of marriage during prepa­ration for Lauren’s big event,” explained the insider. “Being a bridesmaid gave her a raging case of wedding fever.”

After the ceremony, Jessica and Eric flew to the Italian isle of Capri without their kids, 17-month-old daughter Maxwell and 3-month-old son Ace, and she apparently gave him an earful.

“Jessica really chewed him out about getting married,” said the insider. “It was the longest flight in his­tory for poor Eric, but once they got there they seemed to be having a romantic time together.”

Wedding plans made by Eric and Jessica, who was previously married to Nick Lachey, were derailed by her pregnancies. But now that she’s settling into moth­erhood, she’s becoming obsessed with making their family life official with a wed­ding, a source says. And by turning up the pressure on former pro football player Eric, she may finally get her wish, says the source. In fact, there have already been whispers of a May wedding.

“Jessica really took Eric by surprise with her ultimatum,” revealed the insider. “She definitely got his attention!”