By Tim Hepher and Giulia Segreti

PARIS/ROME (Reuters) -Europe's arms firms are ready and willing to step up to fill a defence gap left by U.S. President Donald Trump's pause on military aid for Ukraine, but getting there will take time and depend on how fast political pledges turn into contracts.

Europeans are racing to boost military spending, with European Union leaders set to discuss a proposal to mobilise up to 800 billion euros ($840 billion) at an emergency defence summit on Thursday.

The region's defence firms - which saw a major stock rally on Monday on prospects for more spending - are bullish that they can ramp up production.

But they warned delivery times could stretch into years and said expanding production would depend on governments backing their pledges with spending and fresh contracts.

"Does Europe have the necessary technology to produce the full spectrum of defence equipment that it needs? The answer is yes," Thales CEO Patrice Caine told reporters as the French firm's share hit a record high after forecast-beating earnings.

"It is more a question for buyers, governments and armies... When the contracts come we will be ready, but there is no point in being ready too far in advance."

Countries from the United Kingdom to France and Belgium have pledged to expand defence spending, but there have been repeated complaints from industry that targets do not translate into orders, or that contracts end up going to U.S. rivals.

'WON'T TAKE A COUPLE OF DAYS'

Italian aerospace and defence firm Leonardo is ready to increase production largely via alliances and joint ventures should orders for military equipment rise, the group's CEO Roberto Cingolani said in a results call with analysts.

He said this did not necessarily mean investing in new plants but rather activating unused production capacity, and warned that delivery could be slow.

"Platforms such as main battle tanks or infantry vehicles, aircraft might take two or three years for delivery at the moment," he said. "Other technologies such as sensors, radars, light weapons, satellite services might be much faster."

Emanuele Orsini, president of the General Confederation of Italian Industry, said that defence was a key industry, though it would need a major, coordinated drive to overhaul it.

"We have mature sectors and transforming them won't take a couple of days. It requires industrial planning," he said.

Analysts too have been left pondering how quickly Europe can meet the challenge, given shortages in supply chains and labour capacity already felt broadly by defence and civil aerospace.

"Not immediately, not completely, but eventually possibly," Agency Partners aerospace and defence analyst Sash Tusa said.

Britain on Sunday announced an order for 5,000 lightweight multirole missiles (LMM) from Thales UK for Ukraine, saying this would treble production at its Belfast factory. That follows an order for 650 of the air defence missiles last September.

"For Thales it represents the equivalent of about one year of revenues generated in Britain," which are about 1.5 billion pounds a year, CEO Caine said.

Thales said it is in the process of tripling capacity for defensive radars as well as equipment for the Rafale warplane, while quadrupling the capacity for effectors or the core armament of a missile or weapons system excluding its sensors.

"The ramp-up is not that easy but you have seen our ability to grow our production capacity pretty quickly," Thales CFO Pascal Bouchiat said.

(Reporting by Tim Hepher and Giulia Segreti; Additional reporting by Giselda Vagnoni and Isabel Demetz; Writing by Adam Jourdan; Editing by Benoit Van Overstraeten, David Evans and Catherine Evans)