The schedule for the UK's nuclear reactor building programme has slipped behind already, the safety regulator has admitted, reinforcing concerns that the first reactor will not be built on time.

The Health and Safety Executive (HSE) said it would probably have to issue an "interim" decision on the safety of the two new proposed reactor designs next June, the deadline for its assessment programme. The regulator expects significant chunks of extra work will remain before it can finally approve or reject the designs, but did not say how long this would take.

Kevin Allars, director of the assessment programme at the HSE, said that companies could continue planning and carry out preparatory construction on proposed nuclear sites while they waited for a final decision. But he insisted that construction of a reactor could not start without its consent.

Allars promised there would be no repeat of the chaotic construction in Finland of what was supposed to be Europe's first new reactor in decades. The Areva plant is more than three years behind schedule and more than €2bn (£1.6bn) over budget, with the Finnish regulator trying to approve each component of the design while it is being built. EDF has promised that the UK's first reactor will be operational in 2018, although it had originally said it would be running by the end of 2017.

The HSE said the companies behind the designs – French consortium Areva, EDF and US firm Westinghouse – had been repeatedly submitting information which was incomplete and late.

In turn, the companies are blaming the regulator for not having sufficient resources to carry out the work. The Guardian revealed last year that the arm of the HSE which was carrying out the work – the Nuclear Installations Inspectorate (NII) – had been forced to hire more than a dozen project managers, even though they work for the companies hoping to build the designs under review.

In its quarterly update on the assessment programme, the HSE admitted that this risk of a conflict of interest was a "factor of increasing significance" which it said it would "continue to monitor closely".

Westinghouse, which has put forward its AP1000 reactor design, comes in for particularly harsh criticism. Allars said of the company: "It's very frustrating. We get a load [of work] in late and then we do not get what we were promised or of the quality we were promised. If this carries on they won't get a design acceptance."

The HSE has already raised a red flag over Westinghouse's civil engineering plans for key structures making up the reactor core, which the regulator says are not sufficiently robust. The company was supposed to carry out further analysis by the end of June, but most of this has been delayed, while what had been done "fell significantly short of what we expected".

"Significant issues" are also flagged for Westinghouse's planned control and instrumentation systems to operate the reactor. The company missed a June deadline to provide information on reactor chemistry, "which does not help our confidence that Westinghouse will meet future delivery dates", said the HSE.

A Westinghouse spokesman said: "We accept that some of our input in one or two areas has not met the regulator's expectations."

A Greenpeace spokesman said: "The generic design assessment [GDA] process has already unearthed a string of nasty surprises within the new nuclear reactors' designs. But now we find out GDA won't even be able to give a final green light to the reactor designs. This means we could be faced with the farcical situation where the government is letting utilities press ahead with building work for reactors that haven't been given safety approval."