Gordon Brown is being accused of preparing for a military adventure in Africa after he pledged to provide backing to the Nigerian security forces. His announcement prompted the collapse of a ceasefire in the oil-rich Niger Delta and helped to drive up crude oil prices on world markets.

The Prime Minister's offer to help "tackle lawlessness" in the world's eighth largest oil producer was immediately condemned by the main militant group in the Delta, which abandoned a two-week-old ceasefire and accused Britain of backing what it calls Nigeria's "illegal government". The group issued a "stern warning" to Mr Brown in an emailed statement: "Should Gordon Brown make good his threat to support this criminality for the sake of oil, UK citizens and interests in Nigeria will suffer the consequences."

Speaking at the close on Wednesday of the meeting in Japan of the Group of Eight leading industrial nations, Mr Brown said that the UK was ready to offer the Nigerian military direct assistance to help return law and order to the southern region and to restore oil output.

Download the new Independent Premium app Sharing the full story, not just the headlines

The Prime Minister said: "We stand ready to give help to the Nigerians to deal with lawlessness that exists in this area and to achieve the levels of production that Nigeria is capable of, but because of the law and order problems has not been able to achieve." His comments came ahead of a visit to London by the Nigerian President, Umaru Yar'Adua, next week in which he is expected to appeal for military aid to put down militant groups who have attacked oil pipelines and platforms.

The Nigerian press received the British offer as a declaration of war against rebel groups. The Daily Champion newspaper ran the headline "Battle Line! UK to Declare War on Delta Militants".

Mr Brown is under immense pressure on the domestic front to ease the soaring fuel costs, driven by the global spike in oil prices. Major unrest in the impoverished Niger Delta region has cut the country's capacity to pump oil by one-quarter in recent months, helping to drive oil prices to the record high of $145 per barrel.

However, Mr Brown's initiative appeared to catch the Foreign Office unawares. A spokesman insisted yesterday that there had been "no change in policy" but that "options" were being considered. Senior military sources also said they had been caught by surprise by the decision to offer military aid. There are no contingency plans for intervention in Nigeria that can be activated, they said, and any operation would have to be organised from scratch.

President Yar'Adua came to power a year ago after a controversial election win that was challenged in Nigeria's High Court and contested by independent observers. Despite campaign pledges to tackle endemic corruption, which has raised the country to the top of the global graft index and enriched an elite with illegal oil revenues, the President has made little progress. He has also failed in his pledge to address local grievances in the Delta and restore peace to the region.

A series of attacks on installations and the kidnapping of oil workers by the main militant group, Movement for the Emancipation of the Niger Delta (Mend), has cut Nigerian oil production by one-quarter. The group is demanding a greater share of oil revenues be given to local people as the Niger Delta is among the poorest regions in Africa, despite the immense oil wealth it produces. A spokesman for Mend, Jomo Gbomo, told The Independent that the UK offer was tantamount to a return to colonial policies of divide and rule: "They ought to know better than any other country [not] to involve themselves in any other area aside from development. They [the British] are getting frustrated and we will continue frustrating the oil-dependent markets until justice is offered." Asked if he feared that Nigeria would become the next Iraq or Afghanistan, he replied: "It will not get to that point except if there is foreign interference."

Mend offered to enter peace talks last year but withdrew after the government launched a secret trial against one of its leaders. Attempts to convene a summit have been complicated by the withdrawal of the United Nations envoy who was asked to oversee it, as well as the refusal of Mend to take part.

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

Any action in Nigeria would further stretch British forces. Air Chief Marshal Sir Jock Stirrup, the Chief of Defence Staff, warned the Government last month: "We are not structured or resourced to do two of these things [Iraq and Afghanistan] on this scale on an enduring basis, but we have been doing it on an enduring basis for years. Until we get to the stage when one of them comes down to small-scale, we will be stretched beyond the capability we have."

Defence sources say the only realistic option would be to send special forces along with specialised hi-tech equipment to combat the guerrilla campaign. However, two squadrons out of the four in the SAS are currently deployed abroad, in Iraq and Afghanistan, and one is said to be on exercise. Units of the Special Boat Squadron are also busy in those countries with one contingent working alongside US forces in yet another hunt for Osama bin Laden.

The UK does, however, have special forces in Djibouti alongside other Nato countries in the American-run Horn of Africa task force involved in missions against Islamist militants; some of them can be switched from east to west Africa. It may also be possible to station a Royal Navy warship offshore.

Major General Julian Thompson, a former commander of the Royal Marines, said: "It would be utterly extraordinary to propose anything like a sizeable deployment of forces to Nigeria. Where are they going to come from? The MoD has not exactly got a box marked 'new troops' they can open up for something like this.

"It would be possible to send special forces in limited numbers to help the Nigerian military, but, with the current situation in Afghanistan they cannot be kept there for anything like a prolonged period."

Britain is one of the largest investors in Nigeria. About 4,000 Britons live in the west African country, many working for large companies, including the oil and gas companies Royal Dutch Shell and BG Group.

Click here to have your say