PARIS/SEATTLE (Reuters) - Planemakers Airbus (>> AIRBUS GROUP) and Boeing (>> Boeing Co) have stepped up pressure on French supplier Zodiac Aerospace (>> ZODIAC AEROSPACE) over persistent delays in the delivery of aircraft seats that are disrupting jetliner assembly, industry sources said.

Paris-based Zodiac has been wrestling for months with missed deadlines for seat production, which it blames on its engineering resources being overloaded. A strike last autumn at a Zodiac plant in Texas also affected shipments.

In December, the first delivery of a Boeing 787 Dreamliner to American Airlines slipped into 2015 because of missing seats.

In January, Zodiac said the situation had improved, but that the difficulties facing its seats business were lasting longer than expected and would impact its fiscal first-half results.

Now, leading planemakers are spearheading corrective action, including sending in teams to help tackle the delays, amid new concerns over a Zodiac plant in California that makes seat shells, industry sources said.

The separate measures come as the delays cast doubt on the timing of a number of other upcoming jet deliveries.

Zodiac declined to comment. Shares in the company fell 4.3 percent, their biggest one-day fall since June 1, 2012.

Boeing's most senior production executive told Reuters that Zodiac had failed to prepare adequately for the ramp-up.

"If you pick any business, whether it is aerospace or automotive, where something goes wrong, it is about integration," Pat Shanahan, senior vice president of airplane programs, said in an interview.

"So they over-committed and made changes in their supply chain. Designs weren’t well integrated with requirements. You can’t just point to one thing: the business took off on (them) and (they) didn’t have the capability."

Europe's Airbus (>> AIRBUS GROUP) said it had agreed to work with Zodiac, confirming it would send in engineers. Such coaching is usually a sign of intervention when a supplier goes off track.

"With our suppliers we always take a collaborative approach. Regarding Zodiac, we are sending seat engineers and try to identify solutions, helping them to transform and recover," said Klaus Richter, executive vice president for procurement.

"Zodiac has a long pipeline of seat projects with us and we are configuring our support teams accordingly."

CATALOGUE STATUS QUESTIONED

Behind the scenes, both planemakers are said to be increasingly frustrated over the delays, which coincide with record jet production.

Both manufacturers have held what one source described as "tough" meetings with Zodiac's leadership.

A Zodiac spokesman said chief executive Olivier Zarrouati often visits manufacturers, but declined further comment.

Seats make up about a quarter of Zodiac's roughly 4.2 billion euros annual revenue.

It vies for first place in the seat market with U.S.-based B/E Aerospace (>> B/E Aerospace Inc), with Germany's Recaro a distant third. Shares in B/E Aerospace were up 0.6 percent in a weaker market.

Airlines usually buy seats directly from their manufacturers by following a set of choices approved by the planemakers.

At least one of the top two plane manufacturers has discussed the possibility of removing Zodiac from its seat catalogue, a person with direct knowledge of the matter said.

That would not necessarily deprive it of business, but would mean that any new contracts with airlines had to be approved one by one, using an exceptional procedure that takes more time.

A clear estimate for the length of average delay in the supply of seats was not available, but on the most popular narrow-body jets, the available window for delivery is days.

On larger jets, industry sources say delays in the largest and most costly premium seats, which can cost six-figure sums, can slow deliveries if they turn up out of sequence.

As a last resort, when parts ordered by airlines do not turn up on time, manufacturers have the right to insist on delivering an aircraft empty, which must then be fitted by a third party.

(Additional reporting by Alexandre Boksenbaum-Granier; Editing by Angus MacSwan, Laurence Frost)

By Tim Hepher and Alwyn Scott