Zimbabwe-Rhodesia tonight cut off all shipments of corn to Zambia in a move that could cause widespread food shortages in the drought-stricken and landlocked country.

The action came as talks in London to settle the 14-year-old Rhodesian dispute were at a crucial stage. The Zimbabwe-Rhodesian government has been attempting to pressure Zambia and Mozambique to force Salisbury's guerilla foes into a settlement by attacking economic targets in the two countries.

Patriotic Front guerrillas operate from both Zambia and Mozambique, and continuing guerrilla raids were cited in today's announcement as the reason for cutting off the vital corn shipments to Zambia.

One Western diplomat called the action by Prime Minister Abel Muzorewa's government a "high-stakes poker game" that could have terrible consequences for the people of Zambia.

The source thought it unlikely that any effective pressures could be mounted against South America, Salisbury's main supporter, to make Muzorewa relent.

There was no immediate reaction from the Zambian government of President Kenneth Kaunda to the Salisbury announcement.

Just three weeks ago, Rhodesian commandos knocked out Zambia's northern rail route, the Chinese-built Tazara line to Tanzania, leaving the country to rely on the southern route through Zimbabwe-Rhodesia -- which has now been closed -- for vital corn shipments.

Patriotic Front guerrilla forces under Joshua Nkomo operate from Zambia, while guerrillas under Robert Mugabe have their bases in Mozambique.

Both front leaders, along with Muzorewa, are in London for the talks.

Even before the Salisbury cutoff tonight, Zambia was scheduled to import by next June half its annual corn needs because of drought and low planting last year. More than 200,000 tons -- a third of its requirement -- is being bought from South Africa. This, however, could only come via the southern route.

The government already has cut delivery of corn to mills by 10 percent to conserve its small stockpile. Lusaka, the capital, right now has an acute shortage of mealie-meal, the corn product that is the basis of the Zambian diet.

Over the last several months Salisbury systematically has cut off Zambia's other outlets to the sea. The attack on the Tazara rail bridge, 400 miles from the Rhodesian border, was a particularly disastrous blow.

Previously, Rhodesian planes bombed a rail line through Mozambique and knocked out a road route crossing the Zambezi River into Botswana.

South Africa has closed the other road route through Namibia.

Zambia is left with one road route to the port of Dar es Salaam Tanzania -- thousands of miles away from most of the needed corn.

Because of the bulk of corn, which is known in Europe and Africa as maize, it is impossible to air freight. "It would require the American Strategic Comand to do it," the Western diplomat said. He calculated that about 50 planeloads would have to be flown in daily.

The Salisbury announcement tonight said the government had warned Zambia in a massage Oct. 23 that maize supplies would be cut off unless Zambia took action to prevent guerrilla raids.

The Cabinet statement said that since the incursions had continued, "the government now has no alternative but to cease moving maize to Zambia with immediate effect." It said the cutoff would remain in effect until Zambia stops guerrillas from crossing the border.

There have been reports of heavy guerrilla movements into Rhodesia since the London talks started in early September. The nationalists have been positioning their forces either for an escalation of the seven-year-old war or to get their people into the country to campaign in the election that would follow any settlement.

The Rhodesian announcement only mentioned corn and did not refer to Zambian copper exports, the key to the country's economic survival. The Tazara route had carried 40 percent of Zambia's copper exports. Officials are now hoping to reroute more than 20,000 tons a month on the southern route during the six to ten months needed for the estimated $13 million repair job.

It was unclear what immediate effect the Rhodesian move may have on the London conference. There is little question, however, that attacks on economic targets in Zambia are influencing negotiating tactics of the Patriotic Front leaders in London.

The Front, under quiet pressure from its African supporters, has slowly been making significant concessions in the negotiations, backing away from what seemed immovable positions as little as three months ago.

Speaking before the Rhodesian rail cutoff, one Western diplomat said that if the Front quits the London talks and Britain went ahead with a separate deal with Muzorewa, "Zambia would cave in within two days," and accept the British move. He was perhaps exaggerating only slightly.

"The Zambian economy is on its knees," the source added. "Getting its goods in and out of the country depends on the good will of Rhodesia." Tonight's announcement ended that prospect.

Zambia is surrounded by eight countries, five of which have been embroiled in war, revolution or economic instability for almost two decades.

Zambians often can be heard grumbling about the high price their country is paying for its support of the guerrillas. Many say the only solution is to get the guerrillas out by forcing them to fight Zimbabwe-Rhodesia or to join up with the Front in Mozambique.

This is the kind of informal pressure that has had a bearing in London, where the Front has conceded on a number of points. The Front row has given up a stand long held sacrosanct, the requirement for guerrilla control of the military before a new government is elected.

Publicly, neither the guerrillas nor the five "front-line" African states supporting them will admit the pressure. There is no doubt that it is present.

The other day Mugabe freely acknowledged to a friend that the Front was subject to pressure. He mentioned it as a fact of life rather than objecting, the friend reported.

Zambia first came under Rhodesian attack a year ago in retaliation for the shooting down of a Viscount airliner by guerrillas based here.

The most dramatic attack was a commando operation last April. Helicopter-borne troops landed on a road adjacent to Kaunda's State House residence and destroyed Nkomo's home.

The Rhodesians, with more and better-trained manpower, operate inside Zambia with impunity. To knock out the Tazara rail and road bridges, they had to penetrate almost 400 miles inside the country.

As one Zambian official grimly admitted afterward, "If they can do it once, they can do it again."