Tony Blair will promise today to cut by a fifth the operational nuclear warheads on Britain's Trident submarines, from 200 to 160, the Guardian understands.

The cut is part of the prime minister's campaign to persuade MPs that the government must start work almost immediately on plans detailed in a white paper to build a replacement fleet.

He will personally announce the cabinet's decision in the Commons this afternoon, and is also likely to say that the number of new Trident submarines could be reduced from four to three without reducing the UK deterrent's effectiveness.

The white paper will also say the new Trident system will cost less than £25bn. But it will say this figure represents 5% of the annual defence budget, and about 0.1% of GDP, Whitehall sources said yesterday. Ministers have rejected claims Britain no longer needs nuclear weapons to deter a potential enemy and have embraced the "insurance policy" argument that it is impossible to predict the shape of threats in 20 years.

The promised reduction in Trident, whose warheads will have been halved from 300 since 1997 when Labour came to power, is unlikely to appease critics of nuclear weapons or MPs in all parties who challenge Downing Street's view that Trident must not only be renewed, but that a decision is urgent.

With a public debate and then a Commons vote in February set to follow the white paper, ministers hope they will win the vote comfortably. But they accept they will need Conservative support to push it through.

Up to 40 Labour MPs oppose nuclear weapons, but the key group Mr Blair seeks to persuade are those, including the Liberal Democrats, who think there is no need to take an early decision. The Tories remain pro-deterrent, but their defence spokesman, Dr Liam Fox, said yesterday they would only "replace it [Trident] when necessary".

One Labour minister seemed confident the government would prevail: "There will be some trouble in the parliamentary party. My activists will not want it, but they will not object to it."

The white paper will also reject arguments urging a delay on a decision to commission new submarines by at least five years, as the Lib Dem leader, Sir Menzies Campbell, and many independent analysts have proposed.

Given the long lead times before operational availability - 14 years between the Trident decision and the day it replaced Polaris - it would be too risky. Delay would also not be cost effective, mainly because the nuclear reactors that propel the present boats need replacing soon.

The white paper will say that a sea-based system is the only "credible" nuclear deterrent, rejecting arguments for land-based cruise missiles.

The government has also rejected the argument that a Trident submarine need not be continuously at sea. Instead it will suggest that advances in technology may allow Britain to manage on three rather than four submarines, which would save up to £2bn, one minister said last night.

Anti-nuclear campaigners will step up their protest today. CND and a number of MPs will hand an alternative white paper to No 10 and express concern over the short amount of time being given to discuss the issue.

The Archbishop of Canterbury, Dr Rowan Williams, is also expected today to signal his opposition to replacing the nuclear deterrent.