Airbus CEO Tom Enders poses after Airbus annual press conference on the 2017 financial results in Blagnac near Toulouse Thomson Reuters LONDON (Reuters) - Airbus wants Britain to provide more clarity over the country's future relationship with the European Union, or risk losing investment, the European planemaker's CEO said.

"We must have more clarity on the UK's long-term relationships, not just for the next 20 months," CEO Tom Enders said in an opinion piece in the Financial Times on Wednesday.

"Britain must recognize that future investments are not a given."

His comments come less than a month after Britain's Prime Minister Theresa May agreed a transition deal with European leaders to keep existing trade ties unchanged for 21 months after Brexit, a move seen by politicians as a way of providing businesses with certainty.

But Enders said Britain needed to go further. He called for Britain to remain part of the EU aviation safety certification agency, EASA, to allow Airbus's planes to gain the necessary approvals to let them fly, and said that its business depended on aircraft parts and people being able to move freely between its sites across the UK and Europe.

Airbus employs 15,000 in Britain where it makes all the wings for its commercial aircraft.

Enders also said that there needed to be an agreement covering security, defense and space programs between Britain and the EU, calling for deals to "cement this co-operation".

"It is no good making the right noises. We need a plan that comes from the UK but is also accepted by the EU27," Enders said. "Within these negotiations, the EU will need to offer some flexibility but the UK must also be realistic in its demands; pragmatism must trump pride."

The transition deal, secured by May in March, pushed back the risk of disruption for companies until the end of 2020. But London and Brussels must first agree on their long-term trade ties for the period after 2020 for the transition agreement to become effective.

Airbus, which has its headquarters in France, has 25 sites in Britain and spends more than 5 billion pounds ($7.10 billion) with suppliers in the country. It said that its activities in Britain support 117,000 jobs.

($1 = 0.7041 pounds)

(Reporting by Sarah Young. Editing by Jane Merriman)