Eleven DUP Assembly members have abstained from voting on legislation giving ministers more powers.
The Executive Committee (Functions) Bill was passed before Stormont went into recess on Tuesday.
South Belfast representative Christopher Stalford said his party fully supported it, but his colleague Jim Wells voted no and said some of the party’s members were extremely unhappy.
Trevor Clarke, Alex Easton, William Humphrey and former ministers Mervyn Storey and Michelle McIlveen were amongst those to abstain.
UUP leader Steve Aiken said: “Regrettably today, the Assembly passed what is by any reckoning poor and badly thought-out legislation.
“The fact that the junior ministers admitted that no changes in planning, which this Bill was supposedly about, can be enacted until the revised ministerial code is brought before the Assembly in either the autumn or early winter, shows that not only was there ample time to give this Bill proper consideration, but also that the flaws highlighted in this process re-emphasise that no lessons have been learnt since RHI (renewable heat incentive).
“While we did not manage to defeat the Bill, the fact that some DUP MLAs chose to abstain in person shows the considerable disquiet that there is across much of the Assembly in this process.
“I fear that the flaws in this Bill could haunt us for many years to come.”
The new law was introduced after a court judgment two years ago over a waste incinerator.
It overturned the Executive’s approval for the facility near Mallusk in Co Antrim.
UUP Assembly member Doug Beattie said: “It is important that the Executive is able to operate without fear of being undermined or rocked by a solo run from an individual minister on a controversial policy.”
Defending the fast-tracking of the Bill, Sinn Fein junior minister Declan Kearney said it was to allow decisions to be made on key planning applications.
He said the legislation would help increase co-operation within Stormont’s five-party coalition.
The Bill was passed by 58 votes to 13, with 11 abstentions, and will go forward for royal assent.
The Assembly has been adjourned until September 7.