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A key subcontractor at Wembley stadium has quit, further damaging Multiplex's chances of completing the ground in time for the FA Cup final on 13 May.

AR Security, which was installing security systems and smoke alarms, left the project after encountering payment problems. This raises questions over Multiplex's ability to meet mandatory safety requirements.

The loss of AR is a further blow to the main contractor, which admitted days before the news broke that there was a 30% chance that the stadium would not be ready for the final. AR is the second firm in the past three months to leave because of financial problems. Plumbing contractor SGD went bust at the end of last year.

A source close to AR said it had left the project, where it was due to work until 31 March, after struggling to obtain payment from Honeywell, the main M&E contractor.

The source said: "AR had no choice. The firm will go bankrupt if it doesn't get a settlement on the money it is owed on the project."

The impact of AR's decision on the project is amplified because it will hit Multiplex in the areas that are most likely to delay the scheme.

Multiplex said on Tuesday, before the AR development, that the biggest risks to completion on time included the performance of subcontractors and delays affecting the operational "ramp up" - which is when the systems at the stadium are tested out in series of trial events.

Earlier this week Martin Tidd, Multiplex UK's managing director, admitted that there was a 30% chance the project would not be completed in time. He said it would have to run "like a Swiss watch" for it to be finished for handover on 31 March.

Tidd said Multiplex would give a firm verdict on whether Wembley would be ready in three weeks' time, and added it was putting immense pressure on its subcontractors to get the job finished.

He said: "We have more than 3500 workers out there and there's tremendous pressure on the trade contractors. We are doing as much as we can to manage the trade contractors' performance."

However, he said this would not come at any price. "We will not be held to ransom and we are looking to our guys and the unions to manage things fairly."

Multiplex claimed that the biggest risks were subcontractor performance, design changes, a worsening of industrial relations, inclement weather and problems with the integration and commissioning of M&E systems.

Multiplex said the most risky job was getting the roof finished on time. Work to be completed here includes taking down temporary works supporting the roof trusses that span between the east and west stands and completing the roof cladding.

The biggest threat to work on the roof is high winds. Multiplex said work had to stop if wind speeds exceeded seven knots. Work on the pitch and seating cannot be completed until the temporary works are out of the way.

Multiplex revealed in December that it faces losses of up to £70m on the project.

AR declined to comment and Honeywell was unavailable for comment.

Australian developer Westfield had denied industry rumours that Multiplex is to be replaced as the main contractor at its White City development in west London.