"It is the US's and Europe's interest to prevent nuclear proliferation in a volatile region and to maintain the transatlantic partnership as a reliable and credible driving force of world politics," the MPs wrote.

The MPs from the parliaments of the three European signatory countries said the landmark 2015 deal was a "major diplomatic breakthrough" that had halted the imminent threat of a nuclear-armed Iran.

"We were able to impose unprecedented scrutiny on the Iranian nuclear programme, dismantle most of their nuclear enrichment facilities and drastically diminish the danger of a nuclear arms race," they wrote.

"Not a drop of blood was spilt."

Scrapping the agreement would result "in another source of devastating conflict in the Middle East and beyond," they warned in an open letter published in several newspapers.

Trump has derided the deal as a capitulation to Tehran and has declared it no longer is in the US interest to maintain the sanctions relief his predecessor Barack Obama granted Iran in return for controls on its nuclear programme.

He has demanded that US lawmakers and European allies fix "disastrous flaws" in the deal or face a US exit, possibly as early as next month.

The MPs warned that ending the deal would spell "lasting damage to our credibility as international partners in negotiation, and more generally, to diplomacy as a tool to achieve peace and ensure security."

French President Emmanuel Macron and German Chancellor Angela Merkel will both travel to Washington next week on separate official visits, in part to lobby Trump on the Iran issue.