Britain's Prime Minister Theresa May is flanked by French President Emmanuel Macron and German Chancellor Angela Merkel before their trilateral meeting at the European Union leaders summit in Brussels, Belgium, March 22, 2018. REUTERS/Francois Lenoir/Pool

BRUSSELS (Reuters) - The lights went out on Theresa May as the British prime minister hosted her German and French counterparts for a private meeting at a Brussels summit on Thursday.

President Emmanuel Macron broke into song to lighten the mood.

May faced news photographers, flanked by Macron and German Chancellor Angela Merkel, before talks in the British suite at the European Council building. Just then, it seemed, someone leant on a switch, plunging the room into darkness.

Out of the gloom, came the voice of France’s famously musical leader: “Happy birthday,” he sang. But no cake with candles appeared. Light was restored and media were ushered out.

All three looked grim as they prepared to discuss May’s request for EU support after her accusations that Russia was behind the nerve agent attack on a former Russian spy in England.

Though not formally on the agenda, May had said upon arrival in Brussels that she would discuss Brexit, now just a year and a week away. She will leave after dinner, while the others will return to discuss Britain’s departure over breakfast.

They are due to confirm that Britain can remain effectively in EU structures -- without voting rights -- until the end of 2020 to give business time to adjust and to set out their negotiating terms for talks to start soon on a post-Brexit free trade agreement.