The move comes after AWG, formerly Anglian Water, discovered that it had failed to arrive at the correct value of Morrison Construction when it bought the contractor last year.
Sources claim that final talks have been delayed by at least a week.
O'Rourke's purchase of Laing is now expected to be finalised by the end of this month, rather than in mid-June as first expected.
The firms had intended to complete the deal before Laing's annual meeting.
A source close to the Laing deal said it had had a knock-on effect on the negotiations.
The source said: "The Morrison situation certainly hasn't helped. The due diligence problems over that deal have led both teams to relook at figures. They are saying to themselves 'Let's just check this thing again'."
A City source claimed that Laing was in a vulnerable position in the final negotiations. He said: "By going public on the deal, there is a lack of real alternatives open to Laing. The market expects it to happen so it is under pressure to close it."
But the source added that O'Rourke was in no hurry to sign on the dotted line, and was clearly anxious to know exactly what it was buying.
The delay comes amid reports that the Morrison brothers are considering making a bid for the construction interests that they sold to AWG.
Morrison hasn’t helped. Both teams are now saying ‘Let’s just check this thing again
The water firm last week cut £22.9m from the value of Morrison as it was revealed that six contracts had made a loss at the £510m-turnover group.
AWG said it was not in discussions to sell its construction interests, saying reports "were complete rumour and speculation".
The company revealed this week that it had used its own accountants for due diligence during the acquisition of Morrison because its usual accountant, Pricewaterhouse Coopers, also worked for Morrison.
An AWG spokesperson said the group's accountants had taken advice on construction issues but conceded that the problems with the Morrison contracts had not been identified.
He refused to comment on why the problem contracts were not picked up, saying this could form the basis of any further legal action AWG might take.
He said: "It is not as if we were going in blind to a new area. We made sure we got advice. This is not something you do without back-up."
Construction analysts, however, said it was up to AWG to carry out the due diligence properly, especially as it had no experience of the risks in construction contracts.
One said: "Companies, especially from outside the industry, have to really look at every contract individually. It is a long process, but essential."
Another said the problems thrown up in the due diligence process highlighted the shortcomings of AWG's purchase.