Britain's two major print unions today overwhelmingly rejected a $75 million "final offer" by international media baron Rupert Murdoch to call off their 19-week-old dispute with the four newspapers of his News International Co. here.

The rejection came in a secret union ballot of more than 5,500 former Murdoch employes. It means a probable worsening of the violent clashes between union pickets and police outside the News International plant in East London that have led to numerous arrests of union members and injuries to strikers and police.

The unions, which consider themselves on strike, have been trying to disrupt distribution of Murdoch's newspapers. Murdoch, who took advantage of the strike to accuse workers of walking off the job and fired them, had offered a settlement package.

The offer was based on graduated severance payments of up to $45,000 to each worker based on years of employment. The average payment would have amounted to about $18,000.

Both News International and officials of the larger of the two unions, the Society for Graphical and Allied Trades (SOGAT), seemed surprised by the vehemence of opposition. Although some SOGAT leaders have described the strike as hopeless, and declined to advise members to reject the offer, nearly 3,500 SOGAT members rejected it by a margin of 3 to 2.

Members of the more militant National Graphical Association (NGA), whose leaders had publicly rejected the settlement, voted 4 to 1 against it.

The principal reason for rejection was that the offer amounted to a payoff by Murdoch that did not include negotiations leading to reinstatement of workers in their jobs.

NGA Secretary General Tony Dubbins charged that Murdoch is not interested in negotiations and that he spends most of his time in the United States. Two weeks ago, he said, "Mr. Murdoch flew in from America and within 24 hours laid down a take-it-or-leave-it offer. That's not the way to conduct negotiations."

News International executives reiterated their long insistence that the reinstatement of SOGAT and NGA members in their jobs has never been, and never will be, on the table.

"Those jobs are not going to be available to them," said News International General Manager Bruce Mathews. "So I feel very sorry for the people who have turned down quite a considerable amount of money."

The conflict began Jan. 23, when Murdoch, in a lightning, overnight move, transferred all his operations from Fleet Street, the traditional home of Britain's national newspapers, to the new, computerized facility he had built at Wapping, along the Thames River in the eastern part of the city.

The next day, members of the two unions charged that they had reached no agreement with Murdoch in negotiations over the move, and voted to go on strike. Since then, in view of his ability to operate the Wapping plant with a relative handful of workers, they have charged that he manipulated them into striking to get rid of them cheaply.

Murdoch countered that he had spent years trying to negotiate new technology and necessary job reductions with the militant print unions.