Gordon Brown joined mourners at the Polish embassy in London as they paid their respects to the dozens killed in Saturday's air crash.
Poland's president Lech Kaczynski was among the 96 who died when a plane carrying them to Russia crashed.
Senior government and military figures were on their way to honour 22,000 Polish soldiers who were massacred during the Second World War.
Mr Brown added his name to the book of condolence which was opened at the embassy in Portland Place yesterday.
Mr Brown, who wore a black suit and tie, was greeted by Barbara Tuge-Erecinska, the Polish ambassador to the UK.
He told her the tragedy was "so sad" and urged her to ask for help if there was anything he could do.
Mr Brown, who filled a page with his written message, was accompanied by his wife, Sarah.
She added her own message on the next page as her husband watched solemnly.