Gov. Marvin Mandel "reluctantly" vetoed today the Maryland legislature's latest attempt to reinstate the death penalty.

Mandel said he vetoed the bill, which was designed by the legislature to meet new Supreme Court guidelines, because it was "laced with most serious ambiguities and uncertainties."

Maryland has been without a death penalty since January, when the state Court of Appeals ruled that the old law did not meet the Supreme Court's new strictures. New death penalty legislation will have to await the convening of the 1978General Assembly and the governor's subsequent approval.

Mandel's veto was announced at the final bill signing ceremony of the year and followed by a day the issuance of an opinion by state Attorney General Francis Burch that held that the bill, though constitutional, would present massive procedural problems if implemented.

Mandel criticized the General Assembly for ignoring his proposed version of the death penalty and passing instead an amended bill that "offered an arsenal of weapons for those determined to see that it is never enforced." Burch had approved of Mandel's version.

"I beleive with unequivocal conviction that the death penalty should be part of the Maryland law," Mandel wrote in a letter addressed to state Senate President Steny H. Hoyer and released to the press.

Lt. Gov. Blair Lee III sat in for the ailing governor, who is recovering from an apparent stroke, at the ceremony today, signing into law more than 300 bills and announcing the vetoes - by Mandel - of 580 others.

Lee said he agreed with Mandel's decision to veto the capital punishment bill, saying it was 'very, very unlikely, virtually impossible" that anyone would be put to death as the result of it. Lee said defects by the attorney general would "lead to extended litigation, tying up the courts for years," Lee said.

At least one person in the crowded ceremonial conference room at the Senatehouse applauded when Lee announced that Mandel had vetoed the bill.

Later, Del. Arthur G. Murphy Sr. (D-Baltimore), House leader of the Black Caucus and an outspoken foe of capital punishment, said, "We kept saying in the legislature it was a bad bill."

Del. Nathaniel Exum (D-Prince George's), another Black Caucus member who had participated in the long debate over the death penalty, said the fears expressed by the attorney general about the constitutionality of the proposal "were just the issues we raised" during floor debate.

House Speaker John Hanson Briscoe, who joined Lee and Hoyer in signing the final batch of bills adopted by this year's legislature, said the "world won't come to an end" because of the veto of the death penalty, nothing that no one is under a death sentence in the state.

The decision of the state's highest court to throw out the old affected only convicted mass murderer Anthony Lee Blackwell, the only person found guilty under the outiawed 1975 measure. Blackwell subsequently was resentenced to six consecutive life terms for the firebombing deaths of half a dozen residents of a Baltimore row house.

Mandel wrote to Hayer that "one of the principal reasons that the criminal justice system in general, and the judicial component of that system in particular, is under such heavy attack from the public is its allowance of seemingly endless court proceedings, much of which arises from imperfectly drafted criminal statues." Signing of the death penalty "would exacerbate, rather than alleviate, this problem," according to the governor.

Mandel has until next Tuesday to decide whether to veto a number of still unsigned bills. Among them, and apparently included in bills to be vetoed, are two of the six bills for which veto hearings were held on Monday. One would require local governments to determine an inflation allowance when setting local tax rates, and the other would require carry-out oil for automobiles to be sold in resealable containers.

Among those bills signed by Lee today were the other four proposals that were subjects of veto hearings. One imposes a two-year moratorium on the conversion of existing service stations to gasoline-only facilities; one requires parental notimication before performing an abortion on an unmarried girl under 18; and two extend disability and health insurance benefits to pregnant workers.