In-fighting within Labour ranks intensified last night after government ministers warned Blairite critics of Gordon Brown not to "undermine" the new Prime Minister and to show "unity" and "self-discipline".

As the polls recorded a surge for David Cameron's Conservatives - one placed them on a 15-year high with a lead of seven points at 43% - a Blairite backlash appeared to be gathering momentum with senior Labour figures questioning the grip Mr Brown had on his government in light of recent allegations of blunders, spin and stealing Tory clothes.

Lord Falconer, a friend and confidant of Tony Blair who left the government in June when Mr Brown took over, warned that simply managing crises would not be enough to win the next General Election and that "making clear our vision is the challenge for the Labour Party now".

The former Lord Chancellor added: "If we rely on experience and our ability to handle crises and do not set out our vision for the future of the UK, then we will be offering drift not leadership and the past not the future."

John Hutton, the Blairite Business Secretary, also accepted the PM had yet to convince voters. Denying that the recent slump in Labour fortunes was "the beginning of the end" for the government, he nonetheless said: "The key challenge for us - and Gordon has rightly identified this - is a vision thing. What we have to do now is set out our vision for the next 10 years."

His remarks came as so-called friends of Mr Blair were reported as saying that he was "unhappy" with the direction his successor was taking the party. One was quoted as saying: "Tony feels Gordon should be defending New Labour but the trouble is that when he talks of change, it sounds like he means a break with the past and New Labour."

A triumvirate of ex-Blairite ministers - Charles Clarke, Alan Milburn and Stephen Byers, the respective former Home, Health and Transport Secretaries - were claimed to planning to break ranks with Mr Brown with a raft of policy speeches before Christmas.

However, responding specifically to Lord Falconer's intervention, Liam Byrne, a minister at the Home Office, said the government and party had never been so united and he and his colleagues were driving forward policy on immigration, health, education, housing and crime.

However, he added: "It would be a big mistake for the past - to which we owe so much - to try to undermine the future."

Later, Ivan Lewis, the Westminster Health Minister, said: "What we need now is self-discipline. We need unity behind our Prime Minister and we need to remember that we're Labourites, not Blairites or Brownites."