Gordon Brown is facing demands to make Lord Mandelson foreign secretary in a row that risks tearing apart his government.

The business secretary is secretly pressing Brown to hold a cabinet reshuffle so he can achieve his life-long ambition of running the Foreign Office. Mandelson made the request after he was snubbed for the post of European Union foreign minister at last week's Brussels summit.

Mandelson's reshuffle call puts the prime minister in a perilous position as he struggles to retain the support of the most powerful figures in the cabinet.

If he bows to Mandelson's wishes, he risks alienating David Miliband, the foreign secretary, and his ally Ed Balls, the schools secretary, who is still eager for promotion. If he refuses Mandelson's demand, he risks losing his loyalty with potentially devastating consequences for the election.