Dear Gordon... Tony Blair sends letter to Brown telling him where he's gone wrong

He congratulated Mr Brown on his successes - and spelled out where he thinks he is going wrong.

And he told his friend - and long-time rival - to work on being "post-Blair and not anti-Blair".

The long letter, sent before Christmas, followed a tough first six months in office for Mr Brown.

After the Labour donations scandal, and his apparent U-turn on calling an early election, he has suffered a collapse in the polls.

Downing Street spokesmen yesterday refused to comment on the Premier's private correspondence but it is believed that the letter was accepted by Mr Brown in the supportive spirit in which it was intended.

However, the revelation that Mr Blair has offered his views, even in private, risks infuriating the Prime Minister's supporters.

It could also add to doubts about Mr Brown's ability to lead Labour to a fourth General Election victory at a time when he is fighting to assert his authority.

While he tries to do so, his predecessor has ordered supporters to keep quiet and say nothing which could be seen as disloyal.

The revelation that Mr Blair is still concerning himself with Labour's fortunes will also fuel speculation that he is considering a return to politics.

Since leaving office last June, he has devoted himself to building a career as a diplomat and speaker.

He is estimated to earn around £1million a month from this, but he has recently been tipped for the job of EU president.

The position is currently held by leaders of member states in turn but that will change in 2009 and a full-time position lasting for three years will instead be established.

Mr Blair is the current favourite, although he has not yet declared an interest, let alone announced his candidacy. He would have to give up his outside business interests and his role as a Middle East envoy.

French president Nicolas Sarkozy has reportedly offered to run Mr Blair's campaign. The two men shared a stage at the weekend conference of France's UMP party.

In a speech delivered in French, Mr Blair said the EU could achieve more by acting in unison. "Europe is not a question of left or right but a question of the future or the past, of strength or weakness," he said.

• The man charged with restoring Mr Brown's political credibility was yesterday accused of misleading shareholders in his previous job.

Stephen Carter, the Prime Minister's new chief of strategy, is said to have made "materially false and misleading statements" as a director of NTL, according to court documents filed in the United States.

The allegations emerged following a lawsuit brought by investors in the cable TV firm, which went bankrupt in 2002 with £12billion in debts.

Mr Carter has always denied the allegations, which were made public five years ago. Downing Street refused to comment.