Andrew Grice: Paternity leave: the perfect time to plot a coup

If Tony Blair was hoping that fatherhood would dim Gordon Brown's ambition to succeed him as Prime Minister, he now knows that it was a forlorn hope.

The Chancellor has returned to the political scene after just over two weeks' paternity leave with a big bang that has sent reverberations through Downing Street. The word in Number 10 is that the sleepless nights he admits to following the birth of his son John have given Mr Brown plenty of time to plot his strategy on how to succeed Mr Blair.

On Monday, Brown aides briefed friendly newspapers that he would use a meeting of European Union finance ministers to warn that the proposed new EU constitution could lead to a common tax and fiscal policy. The resulting stories came as a surprise to the Blair camp and to Britain's EU partners, who do not recognise such a threat in the current draft. Nor was the constitution even on the meeting's agenda.

The Chancellor, it seems, was pursuing two objectives: boosting his Eurosceptic credentials with Britain's overwhelmingly Euro-hostile press and creating an "Aunt Sally" that would enable him to claim victory when the agreed constitution does not include what he called "fiscal federalism".

Perhaps there was a third objective: to annoy Mr Blair. If so, Mr Brown certainly succeeded. By talking up the supposed threat from the new EU treaty, the Chancellor appeared to undermine the Prime Minister's repeated assurances that there is no need for a referendum on the constitution.

For good measure, Mr Brown rammed home his point by dashing off an article in the Tory-supporting Daily Telegraph, which ran a front-page lead headed: "Brown the sceptic blasts EU federalism".

The Brown spin machine went into overdrive by telling two friendly journalists that the Chancellor's request to return to Labour's National Executive Committee (NEC) had been blocked by Mr Blair, who is anxious to keep a firm grip on the party's planning for the next general election. For good measure, Mr Brown gave an interview to The Times for publication today.

The media blitz has been dubbed the "Treasury's autumn offensive" in Number 10. What is puzzling Blair aides is that it appears to be directed at the Prime Minister. "Just at the time when we want to be nailing Michael Howard for his past record, we have to waste time dealing with this," one Blairite cabinet minister groaned yesterday.

What is Mr Brown up to? One explanation is that he is merely carrying on where he left off at Labour's conference in Bournemouth, where he staked out a more traditional Labour agenda. This was a pivotal moment in the relationship between New Labour's two founding fathers: although there has been underlying tension since Mr Blair won the party leadership in 1994, it had never broken surface in such a big way.

The Blair camp's view is that the Chancellor is getting impatient for the top job. It suspects Mr Brown is flexing his muscles because he had hoped Mr Blair would stand down this year, halfway through his second term.

Another theory for Mr Brown's moves is advanced by some supporters: that he is kicking up against Mr Blair's decision to bring back Peter Mandelson to his inner circle after the recent departure of Alastair Campbell.

The Blair-Brown partnership is now under more strain than ever before. The differences between the soulmates-turned-rivals is not confined to the EU constitution and Mr Brown's blocking of a euro referendum in this Parliament.

There is also real tension over public services. Mr Blair wants perpetual reform, while Mr Brown believes the money the Government has pumped in must be given time to work. He deeply resents the Blair camp's caricature of him as an Old Labour opponent of change.

Where will it all end? During previous bouts of the "TB-GBs", as the disease is known in Labour circles, the two men have managed to patch things up. "Gordon and I will carry on working in partnership together, as we always do," Mr Blair told a press conference two weeks ago.

Another scenario was painted by the Brown-friendly Daily Telegraph yesterday. "His duel with the Prime Minister, which reached new heights at the Labour conference in Bournemouth, can only end with the political demise of one or the other," itsaid.