BOGOTA, July 7 (Reuters) - Marxist rebels have accused Britain of using big British companies to spearhead a "covert war" in Colombia, according to a letter to Prime Minister Tony Blair published on Tuesday.

In the letter published in the Bogota newspaper El Tiempo, National Liberation Army (ELN) commanders said British firms were using earnings from the oil industry to set up right-wing death squads to kill civic leaders and leftist sympathisers.

The 5,000-strong ELN, Colombia's second largest guerrilla army, said that failure to rein in the alleged covert activities would be tantamount to a "declaration of war."

A spokesman for Blair's office said he was "not immediately aware" of the letter, and the British embassy in Bogota said it was not able to comment.

Blair publicly pledged last month to back future peace talks to end the long-running civil conflict in this Andean country. British companies operating in Colombia have denied any links to ultra-right paramilitary gangs.

The ELN letter singled out British Petroleum Co Plc, the largest single foreign investor in Colombia, and London-based security consultant Defence Systems, accusing them of spending "millions of dollars...to finance paramilitary groups."

Earlier this year, Colombia's chief prosecutor's office wrapped up an 18-month inquiry after finding no evidence that BP was linked to death squads.

"We've not seen anything official (about the letter). But the chief prosecutor gave us a clean bill of health," a BP spokesman in Colombia said on Tuesday. Defence Systems was not available for comment.

Defence Systems helps protect BP's oil operations in the foothills of the Andes mountains in eastern Casanare province.

Last year, BP officials said 120 Defence Systems employees were providing advice on "defensive tactics" but the Colombian military was in overall charge of protection.

The letter was signed by Nicolas Rodriguez, known by his guerrilla name Gabino, and two other leaders of the ELN, which took up arms against the state in 1964 and and regularly attacks oil installations.

"Britain through its transnational enterprises is financing a covert war against the humble inhabitants of our country," they wrote.

"Mr. Minister, our people await an explanation and corrective action," they said. "Failure to do so means your government cannot control British companies and at worst we will consider it as a subtle declaration of war."

ELN leaders are preparing for talks in Germany this weekend with a group of Colombian business, religious and social leaders intended to pave the way for peace negotiations.

More than 35,000 people have died in the conflict in the last 10 years and more than a million peasants have been forced to flee their homes as a result of the violence.

The British army has in the past helped train the Colombian military, including its elite Special Forces unit which carries out anti-drug and counterinsurgency operations.