The prime minister was leaving a wreath-laying ceremony with Belgian Prime Minister Charles Michel when a traffic collision happened near NATO’s headquarters in Mons.
A Belgian government source told Sky News the leaders were travelling in separate armoured Mercedes.
Another car entered the convoy around them without instructions, causing the motorbikes to manoeuvre and crash into each other.
Mr Michel, who is a motorcyclist himself, got out of his car and knelt on the road to check the police officers’ condition.
The two motorcyclists were taken to hospital but no-one else was injured.
Earlier, Mrs May had been in Saint-Symphorien to visit the graves of the first and last British soldiers to be killed in World War One.
John Parr of the Middlesex Regiment died on 21 August 1914, while Private George Ellison of the Royal Irish Lancers was killed on the Western Front on 11 November 1918 – 90 minutes before the armistice came into effect.
In the note left by the resting place of Private Parr, Mrs May quoted a line of wartime poetry The Soldier, written by Rupert Brooke.
She wrote: “There is in that rich earth a richer dust concealed.”
At the grave of Private Ellison, also in blue pen on a headed Downing Street card attached to the garland of poppies, Mrs May wrote: “They were staunch to the end against odds uncounted … We will remember them.”
The line was from a poem written by Laurence Binyon and published in September 1914, which is often quoted in Remembrance Sunday services.
Mrs May later travelled to Albert, the town at the heart of the French Somme region, where she was seen smiling and waving with President Emmanuel Macron.
The leaders held a private meeting and working lunch, before they are due to depart for a wreath-laying ceremony at the nearby Thiepval Memorial.
The site bears the names of more than 72,000 members of the armed forces who died in battle and holds an annual commemoration for the Missing of the Somme.