Former President Ford said yesterday that President Carter will have to veto increased spending by an "irresponsible" Democratic Congress or face "very serious" inflation within the next two years.

In his first lengthy meeting with reporters since leaving the White House in January, Ford broke his self-impose moratorium on criticism of his successor to say that Carter is on the verge of "losing the battle with inflation."

Ford, who had a one-hour Oval Office meeting with Carter Thursday, said the new President has "a concern" about inflation, "but it's not reflected in his budget."

He said the "real danger" is not in the relatively modest increases Carter has approved in current government spending but in the "tremendous increase" in recommended authority for future appropriations.

Carter has suggested a $28.1 billion addition to budget authority in fiscal 1977 over the sums recommended by Ford in his january budget, and an additional $26.8 billion increase in 1978.

Saying Congress appeared eager to increase those amounts, Ford warned: "if they don't do something to get a handle on the fiscal situation, they're going to have a tough time by 1979. That where the real crunch will come."

At the same time, the former President criticized Carter's effort to cut off funding for several dozen water projects as impractical and unwise. "The same OMBS [Office of Management and Budget] people who gave those projects to Carter gave them to me, and I said, 'you're wrong'" Ford remarked. "I think I've been proved right."

In a spirited 75-minute exchange over breakfast with a group of 40 reporters, Ford also said:

' - there are risks in Carter's aggressive championing of human rights, but it is too soon to tell how the Soviets wiil really react to it.

His own efforts to secure a strategic arms agreement with the Soviets last year were hampered because "we had less flexibility than we needed" from the Pentagon.

He would not disclose how much money he was making from his television, book-writing and other activities, but said they were all "constructive," and "that's what the free enterprise system is all about."

He wanted to be "available" for another White House try "if it's helpful" to the Republican Party, and will not make a decision on whether to run until after the 1978 mid-term elections.

He opposed Carter's plan for "universal" voter registration, because he said people should be willing to make two trips to the courthouse to register and vote.

The former President, back in Washington for three days, appeared totally relaxed as he puffed his pipe and answered reporters' questions before heading off for a golf date at Burning Tree.

He said he has a "very friendly, very easy" conversation Thursday with the man who beat him last November. He remarked with pleasure that "Ben Franklin's picture is still on the wall" of the Oval Office, where Ford had left it.

He declined to be drawn into any criticism of Carter's continuing campaign for public support, or his handling of congressional relations or his appointments.

But Ford said he was "seriously concerned that some of the fiscal decisions of the new administration and even more in Congress . . .will lead us to losing the battle with inflation."

Claiming an "excellent trend" in reducing inflation in his last year as President Ford said, "If the majority in Congress is not check-reined, we could have a very serious fiscal situation" before the end of Carter's term.

He said Carter had erred in recommending such large increases in future-year budget authority and now Congress was threatening to boost even those totals. "He can and he must veto some of those bills," Ford said. "I think he can get support in the country and be sustained on that."

But when a reporter asked if Ford applauded Carter's efforts to economize by stopping some 32 dam and reclamation projects, the former President said he had rejected "the same package when they submitted it to me."

"I had sone understanding of what Congress would do," he said, "and they're doing it." The Senate has already amended the public works bill to prevent any cutoff of funds for these projects.

"In addition," Ford said, "I thought these projects were better for employment than some make-work projects" that might be started under a new public service-jobs bill.

"My position was that we should finish the projects already begun and tell Congress there will be no new starts. I think that would have worked," he said.

On the questions of Carter's human-rights offensive, Ford was rather guarded. He said his administration had been "quite successful" in aiding dissidents within the Soviets Union by quiet diplomacy. "Whether he will be, we don't knos. We've only seen the tactic we haven't seen any results."

The former President said there was "a possibility" the campaign could "interfere with the SALT talks or the talks on MBFR (mutual amd balanced force reduction in Europe) or have the effect of solidifying the Warsaw Pact nations."

But he said the question was "in "limbo" at the moment, and Soviet party leader Leonid I. Brezhnev's speech last week, calling Carter's statements on human rights "unacceptable," was perhaps no more than what Brezhnev had to do politically for his own foreign and domestic audience.

At the same time, Ford conceded publicly for the first time that "inflexibility" in the Pentagon was one factor in preventing him from negotiating a new SALT agreement with the Russians last year.

He denied that his challenge from Ronald Reagan for the Republican presidential nomination inhibited his bargaining position, but said the Pentagon was a problem for him.

"You had to get everybody on the team, and on some of those matters, the Pentagon felt very strongly . . . and I assume it still does," he said.

Ford declined to spell out the specific issues, saying they were "very sensitive" and might affect the current Moscow talks. But it is known that Pentagon officials fought proposals from former Secretary of State Henry A. Kissinger to bypass the issue of the Russian. Backfire bomber and the U.S. cruise missile by treating them both as if they were short-range weapons, not part of the inventory of strategic weapons-carriers to be limited in the treaty. Carter has also suggested excluding these weapons from a new Soviet-American pact.

When a reporter asked Ford if he could not simply order pentagon compliance, he said, "You can say that, but then things start to leak out here and there. It's better to have everyone's agreement, and we couldn't get that."

Ford displayed some sensitivity to questions on his personal finances, saying they were "a private matter. I'm a retired public official and I don't intend to discuss it."

Since leaving the White House, Ford has signed contracts with NBC for several documentaries and with a book publisher for his memoirs, as well as taking several foundation positions and short-term university teaching assignments.

"As long as they are constructive," Ford said of his projects. "I will do them. The money side is for my agents to work out.If the money comes, fine. That's what the free enterprise system is all about. But I'm deciding on the basis of their being constructive projects."

While describing himself as "retired," Ford also said he did not rule out another try for the presidency in 1980. He said he would sdecide "shortly after the 1978 election," and would consider the record of Carter and the Democratic Congress, the results of that election and whether "other Republicans are emerging. I hope they do," he added.