The tough message, delivered on Breakfast with Frost this morning, is his first public riposte to party figures whose sniping at the leadership has triggered a slide in Labour's ratings.
Aides in the Blair camp complain that grumbles about Labour having ditched its policies to become an "SDP Mark 2" are an uncanny echo of the propaganda line emerging from Conservative Central Office.
Mr Blair will reject allegations that Labour has no policies, and insist on pressing ahead with modernising the party.
He believes Labour must not repeat the mistakes of the past, when any leader who tried to advance, change or broaden support was accused of abandoning traditional support or values.
Refusing to apologise for the pace of change, he will argue that the party has a choice - as it did over the abolition of the old nationalising Clause IV - whether it agrees with him or not.
By opting for a philosophy of vast social and economic revitalisation, Labour has become a party "that runs with the grain of human nature," he claims.
His appearance on Breakfast with Frost today will be followed by a whistle- stop three-day tour of Ireland, north and south, where he will meet the Irish Prime Minister, John Bruton, and senior Ulster politicians. Mr Blair is to spell out that Labour generally supports John Major's Northern Ireland policy, particularly on the decommissioning of IRA arms.
Next week, the Labour leader will take his uncompromising message to the Trades Union Congress in Brighton. In his first speech to the TUC, he is to warn union leaders that Labour will not be dictated to on the level of the national minimum wage to be implemented by a Blair administration; nor committed to a figure or a formula before the party takes power - and only then after consultations with employers and other groups.Reuse content