Tony Blair sent a personal message to Bill Clinton wishing him well amid reports that he was admitted to hospital for heart surgery.
The pair developed a close friendship and political ties
The prime minister sent his best wishes to the former US president, his friend and political ally.
Mr Clinton, 58, is to undergo heart bypass surgery at the Columbia Presbyterian Hospital, New York, after suffering chest pains.
A Downing Street spokesman confirmed a "personal message" was sent.
Mr Blair suffered from heart problems himself last year.
US news networks said Mr Clinton went to a hospital near his New York state home after recently suffering from chest pains.
He was then transferred to the hospital in New York City.
According to ABC News, Mr Clinton may undergo quadruple bypass surgery at the hospital, which is considered one of the leading institutions in the US.
The two-term former president has lost a lot of weight since leaving office in 2001 and friends have said he has been dieting and exercising in recent months.
He has never been far out of the public eye since leaving the White House.
He was warmly received at the Democratic National Convention in July
His autobiography My Life, which he has been promoting, has also ensured that he has remained a high-profile figure.
Last year Mr Clinton became embroiled in controversy over Mr Blair's own heart problems when he said the prime minister had discussed a heart condition with
him several years ago.
However Mr Blair insisted the irregular heartbeat he suffered last October was a one-off.