Western powers to study options if Syria chemical weapons used again-UK

* Raids trigger heated debate over legality

* Assad in 'good mood' - lawmakers

* UK's Corbyn says legal basis questionable

* Assault denounced by Damascus and allies

* US 'locked and loaded' - Haley

By Elizabeth Piper and Tom Perry

LONDON/BEIRUT, April 15 (Reuters) - Western powers will study "options" if Syria's government again uses chemical weapons, but nothing is planned as yet, Britain's foreign minister said on Sunday, after raids on Syrian targets triggered heated debate over their legality and effectiveness.

U.S., French and British missile attacks struck at the heart of Syria's chemical weapons programme on Saturday in retaliation for a suspected poison gas attack a week ago, although the restrained assault appeared unlikely to halt Syrian President Bashar al-Assad's progress in the seven-year-old civil war.

The bombing, denounced by Damascus and its allies as an illegal act of aggression, was the biggest intervention by Western countries against Assad and his powerful ally Russia.

But the three countries said the strikes were limited to Syria's chemical weapons capabilities and not aimed at toppling Assad or intervening in the civil war.

Speaking on British television, British Foreign Secretary (Minister) Boris Johnson threw his weight behind Prime Minister Theresa May's decision to take part in the attack, saying it was the right thing to do to deter further use of chemical weapons.

"There is no proposal on the table at the moment for further attacks because so far thank heavens the Assad regime have not been so foolish as to launch another chemical weapons attack," he told the BBC's Andrew Marr show.

"If and when such a thing were to happen, then clearly with allies we would study what the options were."

His comments appeared in line with those of U.S. ambassador to the United Nations, Nikki Haley, who said at an emergency meeting of the U.N. Security Council that Trump told her that if Syria uses poisonous gas again, "The United States is locked and loaded."

The Western countries said the strikes were aimed at preventing more Syrian chemical weapons attacks after a suspected poison gas attack in Douma on April 7 killed up to 75 people. They blame Assad's government for the attack.

But British opposition leader Jeremy Corbyn said that the legal basis used to support the British role was debatable, adding that he would only support action backed by the United Nations Security Council.

"GOOD MOOD"

"I say to the foreign secretary, I say to the prime minister, where is the legal basis for this?" Corbyn said.

"The legal basis ... would have to be self-defence or the authority of the U.N. Security Council. The humanitarian intervention is a legally debatable concept at the present time," he said in an interview with the BBC on Sunday.