Angered by what Manassas Park officials said is an unjustifiable cable rate increase, the City Council last week threatened to revoke its cable company's 15-year franchise contract unless service improves.

Cablevision Inc., which provides cable television to 14,000 customers in Manassas, Manassas Park and western Prince William County, has announced it will raise the basic monthly rate 21 percent, from $13.95 to $16.90, beginning March 1. Rates went from $8.75 to $13.95 a month last June, an increase of 59 percent.

"If people are going to pay the increased rate, the company should offer good service," said Manassas Park City Manager Jerry W. Davis. "If they can't do that, they should let us know and then we can bring in another company." The contract expires in 1998.

Officials with the cable company said the rate increase was necessitated by rising labor, administrative and programming costs. Since winning the right to operate in Manassas and Manassas Park, the company has lost money every year, including $900,000 last year, officials said.

The seven-page letter, signed by Manassas Park Mayor G. Robert Maitland and unanimously endorsed by the City Council, notes numerous violations by the cable company of provisions in the contract and subsequent cable ordinances adopted by the city.

R. Calvin Sutliff Jr., president of Cablevision Inc., said he agreed with many of the issues raised in the letter and said he is "making all the corrections."

"We're very sensitive about the complaints," Sutliff said. " . . . We're really trying hard to look for a solution."

Many of the violations addressed in the letter pertain to lack of cooperation in submitting financial reports and complaint logs and not providing certain services.

In addition to meeting with Manassas Park officials next month, Sutliff said he will appear before the Manassas City Council Monday to "answer all the questions asked of me."

Subscribers were particularly upset by the rate increase announcement because it was made two months after two independent stations were canceled and less than a year after the last rate increase.