The Conservative party see David Miliband as an increasing rival to David Cameron, as the environment secretary moves to enhance his media profile.
According to reports, the Tories have launched a 'stop Miliband' unit dedicated to tracking his movements. Documents passed to the Daily Telegraph show a small team is now monitoring his blog, media appearances and speeches.
It has already emerged the Tories are monitoring Mr Miliband's blog after they complained he had used his department blog to defend the Labour party's environment credentials.
"We would be crazy not to be watching him. It is obvious that a challenge by Miliband cannot be ruled out," a party source told the paper.
At 41 years old, a challenge from Mr Miliband would deprive Mr Cameron of the 'youth card'. As environment minister he could also match the Conservative leader on green issues, while he is has the advantage of not being associated with the Conservative economic policies of the 1980s.
Labour party members are increasingly calling on Mr Miliband to challenge the chancellor Gordon Brown for the Labour leadership when Tony Blair leaves Downing Street, most likely after the May elections.
Charles Clarke, who was sacked as home secretary by Mr Blair last year, has publicly urged the environment minister to stand, claiming there is "no broad consensus" within the Labour party for a "Brown coronation".
Speaking to the Royal Television Society in London, Mr Clarke said: "The leadership is not a done deal to be sorted out within our party and then delivered to a grateful nation in a gleaming package. A genuine and open political contest may well be necessary."
Following weekend reports Jack Straw will be coordinating Mr Brown's leadership campaign, Mr Clarke added MPs had damaged Mr Blair's credibility by announcing their leadership and deputy leadership bids.
Mr Miliband has not publicly ruled himself out of the leadership race. Although he has described Mr Brown as an "excellent prime minister in waiting" less flattering comments have been used to the Conservatives' advantage.
Last month, Mr Miliband appeared to say Mr Brown would be an unpopular leader when he told BBC One's Question Time: "I predict that when I come back on this programme in six months' or a year's time people will be saying, 'Oh, wouldn't it be great to have that Blair back, we can't stand that Gordon Brown'."