His intervention came amid suggestions that ministers could be ready to reduce it, although the Government insisted no changes were imminent.
Critics have warned that as the roll-out of the flagship welfare reform gathers pace, the six week wait is contributing to rising debt, rent arrears and evictions.
They have called for it to be reduced to four weeks to bring it in line with salary payments.
Dr John Sentamu, the second most senior member of the Church of England, said the waiting period is leaving millions of people who are already in debt with "nothing to fall back on".
It came after a Tory MP who threatened to rebel over the roll-out of UC, Stephen McPartland, said he believed the critics were "very, very close to getting a resolution" on calls to cut the waiting time to four weeks.
Theresa May avoided a Tory revolt on the issue in a House of Commons vote on Wednesday after making a concession by committing to scrap charges of up to 55p a minute to call a UC helpline.
She controversially ordered her party to abstain on a non-binding Labour motion calling for the introduction of the reform to be paused.
Writing in the Sunday Times, Dr Sentamu said: "In the Bible, the hardest-pressed of all poor people were summarised as 'widows and orphans' for they were the group most at risk and with least support.
"Our concern should be for their present-day successors whose essential outgoings are costing more and more and their incomes standing still or going down.
"They fear Universal Credit, particularly because it seems to assume that everyone has a nest egg which will tide them over as they wait a minimum of 42 days for payouts.
"That is grotesquely ignorant, for millions of people, especially those in need of support, are already in debt and have nothing to fall back on."
Meanwhile, former chancellor George Osborne was blamed in a new analysis for undermining UC by making cuts to its work allowance element, which is the amount families can earn before their support is withdrawn.
The 2015 summer Budget cuts will put an estimated 340,000 additional people in poverty by 2020/21, according to the Joseph Rowntree Foundation.
This equates to a single parent working full-time on the National Living Wage being Â£832 worse off by 2022 than they would have been had the cut not been made.
The equivalent figure for couples with a single breadwinnner working full-time is Â£468, prompting the charity to call for the work allowance to be restored to its original level, which would cost Â£1.2 billion for single parents and Â£900 million for couples with children.
Commons Work and Pensions Committee chairman Frank Field said the Prime Minister must reform UC or face the Conservative Party being seen again as "the nasty party", a phrase coined by Mrs May in 2002.
Writing in the Mail on Sunday, Mr Field said: "Unless she acts fast to defuse the political timebomb that is Universal Credit, she risks undoing many of the good things she has done. The Conservative Party will be seen as the nasty party once again.
"I doubt Mrs May would have approved it in its current form had she been in Number 10 at the time. She will have seen by now that the six-week period of limbo under Universal Credit is pushing families to the brink of destitution.
"To avoid any more political carnage on the home front, she needs to put it right. Fast."
A Government spokesman said: "The Government remains determined to ensure that people joining Universal Credit don't face hardship which is why we recently announced significant improvements to the system of advance payments that people can get get as soon as they get into the system.
"As repeated this week, we will continue to monitor and take any actions if necessary. But no decisions or announcements on any further actions are imminent."
Johnny Mercer, who was among a group of Tory MPs to meet Mrs May over Universal Credit last week, called the benefit a major tool in tackling poverty and said it was working in getting people into employment.
"I think the phone number was a bit of an error, really, and the seven-day wait that's built in, making it six weeks before someone gets their first payment, I think is quite difficult to ask people to do," he told Sunday with Paterson on Sky News.
"My concerns around this have always been the same, particularly the two issues that I've just mentioned to you there.
"And yes, we have seen movement on the phone line, which is good, we may see movement on four to six weeks, no one has given me that steer, but at the end of the day this is a modern, compassionate Conservative Party that wants to get these things right."
He refused to be drawn on any assurances he had been given during his meeting with Mrs May, saying it was a private meeting.