Illinois' steady recovery from the heartland's lingering recession was dealt a blow last month when unemployment rose 1.8 percentage points to 9.5 percent, the largest monthly jobless rise ever recorded here.

Figures for February released by the U.S. Labor Department today show that the number of jobless workers in the Prairie State rose by 105,000 to 543,000, reversing several months of improvement in that category.

The jobless rise here helped fuel a sharp February jump to 7.3 percent in the national unemployment figure from 6.7 percent, the biggest monthly increase in six years in the rate of idled U.S. workers.

"We were startled, we had expected a 1.1 percent decrease in unemployment," said Lynn Pierce, spokeswoman for Sally Ward, the director of the Illinois Department of Employment Security.

Pierce traced much of the rise in the jobless rate to layoffs by retail merchandisers of extra sales staff after the busy Christmas holidays and post-holiday sales. The trend was strengthened by cutbacks, employe furloughs and factory closings among heavy-equipment and electronics manufacturers, including Deere & Co., American Telephone & Telegraph, Ford Motor Co. and LTV Corp.

In addition, the state's recent progress in economic recovery "encouraged more people to re-enter the job market," she said. This added to the growth of the statistical pool of those without work. Pierce said the figure of 105,000 additional jobless is composed of a rise of 28,000 in job seekers, together with a loss of 77,000 jobs.

However, she said, the general trend for Illinois continues to reflect expansion of the state's economy. She said there are 5.1 million jobholders in the state, 23,000 more than in 1985, 75,000 more than in 1984 and 257,000 more than in 1983.

The employment security department has traced a slow decline in the annual jobless rate, from 9.1 percent in 1984 to 9 percent last year, and a projected 8.7 to 8.8 percent rate overall for 1986, she said. "The labor force and employment will increase in 1986, we think," she said.

"Sometimes, we are confronted with statistical anomalies on a month-to-month basis," she added, saying there have been reporting deviations as a result of changes in Labor Department survey techniques. This led to a "slight understatement of January unemployment and, hence, overstatement of February's figures," Pierce said.

For Republican Gov. James R. Thompson, who is seeking election this fall, the news was unwelcome. "The governor is disappointed," said Assistant Press Secretary Jim Bray. "January's figures showed a 7.7 percent unemployment rate but, at the time, the governor cautioned we should not get too optimistic."

Some prominent Illinois Democrats chorused warnings of "drastic reductions in income tax and sales tax revenues," as State Treasurer James H. Donnewald put it.

Thompson and the legislature have launched a $2.3 billion program to spur the economy. Included are low-interest business loans, and improvement projects for highways, sewers and other aging components of the state's infrastructure.