Sprint Nextel Corp. shares fell Tuesday after two of its rivals cut their cell phone service rates and an analyst downgraded the wireless carrier, saying Sprint will almost certainly need to follow suit to prevent more customers from defecting to other carriers.

Sprint shares fell 15 cents, or 4 percent, to $3.67 in afternoon trading. The stock has ranged from $2.03 to $5.94 over the past year.

On Friday, Verizon said it would cut the rate it charges for its unlimited monthly calling plan by $30 to $70 for so-called postpaid customers, who sign annual contracts. Postpaid customers tend to spend more than prepaid customers, who pay on a month-to-month basis.

Later that day, AT&T Inc. announced the same price cut.

The companies added that they would charge $100 for an unlimited data and voice plan, rather than the $130 they charged previously.

Also last week, MetroPCS Communications Inc. said it would revamp its calling plans to include all taxes and fees in the monthly listed prices, which now start at $40 for unlimited calling, texting and Web access. Previously, plans started at $30 but did not offer as many features, and the price did not include fees and taxes.

Sprint charges $100 monthly for a plan that includes unlimited voice, data and texting. Through its prepaid Boost Mobile service, it offers an unlimited plan that includes taxes and fees for $50 a month.

In a client note Tuesday, Sanford C. Bernstein analyst Craig Moffett lowered his Sprint rating from "Market Perform" to "Market Underperform." Moffett said Sprint "had it easy" last year, as Verizon and AT&T — the largest and second-largest cell phone carriers in the country, respectively — didn't make any major reductions in postpaid cell phone plan prices.

Moffett said the recent changes could hurt Sprint, though, as its cost structure makes the company "poorly suited" for competing in the latest pricing skirmish.

"With a cost structure built for competition at the high end, but with pricing plans increasingly geared to prepaid, Sprint looks increasingly stuck in an untenable middle," he wrote.

The analyst said that his analysis shows Sprint's cost per subscriber was almost $36 in the third quarter, compared with $29 at AT&T, $28 at Verizon and $32 at T-Mobile USA Inc.

He also thinks it will be hard for Sprint to meet expectations for growth in prepaid customers, saying that with a 6.7 percent third-quarter churn rate — the percentage of subscribers dropping service — Sprint needs to add more than 1.1 million gross prepaid customers each quarter to stay where it is now.

However, Moffett raised his price target for the stock by 50 cents to $3, mostly due to Sprint's stake in wireless networking company Clearwire Corp.

Sprint spokesman Scott Sloat said the company does not comment on analyst reports. Regarding the price cuts, he said his company's unlimited plan is still cheaper than Verizon's and AT&T's because it includes texting — something that costs and additional $20 per month at those carriers.