NEW YORK >> United Airlines is getting tough on passengers with oversized carry-on bags, even sending some of them back to the ticket counter to check their luggage for a fee.

The Chicago-based airline has started a push to better enforce rules restricting the size of carry-on bags — an effort that will include instructing workers at security checkpoint entrances to eyeball passengers for bags that are too big.

In recent weeks, United has rolled out new bag-sizing boxes at most airports and sent an email to frequent fliers, reminding them of the rules. An internal employee newsletter called the program a "renewed focus on carry-on compliance."

The size limits on carry-on bags have been in place for years, but airlines have enforced them inconsistently, rarely conducting anything beyond occasional spot checks.

United says its new approach will ensure that bags are reliably reviewed at the security checkpoint, in addition to the bag checks already done at gates prior to boarding.

Passengers are typically allowed one carry-on bag to fit in the overhead bin, which can be no larger than 9 inches by 14 inches by 22 inches. Fliers can also bring one personal item such as a purse or laptop bag that fits under the seat in front of them.

People flying with oversized bags can have the suitcase checked for free at the gate, a longstanding practice. But those who get halted at the entrance to security must now go back to the ticket counter and pay the airline’s $25 checked-luggage fee.

Some travelers suggest the crackdown is part of a larger attempt by United to collect more fees. The airline says it’s simply ensuring that compliant passengers have space left for them in the overhead bins. In recent years, the last passengers to board have routinely been forced to check their bags at the gate because overhead bins were already full.

"The stepped-up enforcement is to address the customers who complained about having bags within the size limit and weren’t able to take them on the plane," United spokesman Rahsaan Johnson said. "That is solely what this is about."

It has nothing to do with revenue, Johnson said, adding that one non-compliant bag takes up the same space as two compliant ones.

But the airline is likely to benefit financially if more passengers are turned back at security.

"This new program is primarily to drive new revenue and will likely delay the boarding process even more unless better education is provided around what is and is not acceptable," said Brian Kelly, an industry watcher who writes about flying trends at ThePointsGuy.com.

But, he added, having fewer bags on board could also be good for passengers.

"I’ve been whacked more times than I can count by people loaded down with their life’s worldly possessions," Kelly said.

United collects $638 million in checked-bag fees a year but wants to increase that figure. In a January earnings call, the airline’s chief revenue officer, Jim Compton, said United hopes to collect an extra $700 million over the next four years from extras such as baggage fees and the sale of extra legroom.

Those fees have helped the airline industry return to profitability even as the price of fuel has climbed. While airfare has risen faster than inflation, it could have risen faster still without the added revenue.

Other airlines have bag sizers at checkpoints, but enforcement was sporadic at best.

American Airlines asks staff at some of its largest airports "to do an eyeball test" of carry-ons. The airline has even used tape measures to enforce polices.

Delta Air Lines puts agents near security to look for oversized carry-on bags "during peak times at hubs and larger airports." It has also improved technology to check bags faster at gates.

United is going further than other airlines. Its bag sizers have a space for bags going in overhead bins and another for those items going under the seats.

Christina Schillizzi, a frequent United flier from New Jersey, said she was shocked to see the flight crew stringently forcing people to check carry-on bags on a recent flight. They even questioned if her laptop would fit under the seat.

"Fliers were naturally annoyed" and did not want to give up their luggage, she said. "Ultimately, the less-than-friendly flight attendants won out."

United has also updated its website, telling passengers to use the new sizers to test their luggage "so you can check any bags that are too large right there in the lobby."

"You may have purchased a bag that claims to be ‘official carry-on size,’" the airline cautioned. "However, this labeling can be misleading because it doesn’t specifically represent United’s size restrictions."

The process of getting on a plane dramatically changed in 2008, when U.S. airlines started charging extra to check a suitcase. To avoid the fee, more passengers started bringing suitcases into the airplane cabin, many of them overstuffing the bags. Suddenly there was not enough room in the overhead bins.

Airlines now sell priority boarding passes guaranteeing those who pay extra get some space in the overhead compartments. Everybody else is left jockeying for a position at the gate, hoping to get on board before the bins filled up.

Once on the plane, passengers take longer to sit down because they are trying to cram over-packed suitcases into the already overflowing bins. Airlines have been installing new, larger overhead bins, but it has not entirely solved the problem.

"It was getting out of control with how much people were bringing on board," said Michel Jacobson, a frequent United flier who works for a Washington D.C.-based trade group.

Jacobson isn’t so worried about paying the $25 checked-bag fee — it’s waived for him as an elite member of United’s frequent-flier program. Instead, he fears needing to show up at the airport earlier to check a bag he’s used to bringing onboard.

When Spirit Airlines started charging passengers in 2010 to place bags in the overhead bin — something only Spirit and Allegiant Air do — executives said the move helped improve on-time performance. Spirit charges $5 more for carry-on bags than checked bags.

Last year, United reconfigured its gate areas to separate the people in boarding group 1 from those in group 2 and group 3 and so on. The goal was to instill some order and speed up boarding.

Then on Feb. 21, Aaron Goldberg, United’s senior manager of customer experience planning, notified frequent fliers that the airline was launching "a broad communications campaign to support awareness of our carry-on baggage policy."

And for those fliers with non-compliant bags there was a link offering discounts — and the ability to redeem frequent-flier miles — on suitcases from Tumi, Samsonite and Hartmann.

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Scott Mayerowitz can be reached at http://twitter.com/GlobeTrotScott .