The Prime Minister of Belize warned David Cameron last night that he risks damaging relations between Britain and the Central American country if he gives Lord Ashcroft an influential position in a Conservative government.

Dean Barrow said his government was in a "state of war" with the billionaire Conservative deputy chairman over his business interests in the country.

In an extraordinary intervention in the margins of the Commonwealth heads of government summit in Trinidad and Tobago, Mr Barrow said he hoped no "institutional harm" would come to British-Belize relations, which he described as valuable, as a result of Lord Ashcroft's role in a Tory government. Britain has a military training base in the country and Belize is a strategic ally in the region.

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"It's not for me to presume to advise Mr Cameron," Mr Barrow said. "All I can do is to say that I would hope that a practical problem would be managed in such a way as not to damage relations between Belize and the UK."

The Tory peer is ploughing millions of pounds into marginal constituencies and has been tipped for a foreign affairs role if the Tories win power.

Mr Barrow has been a fierce critic, claiming that Lord Ashcroft operated a monopoly by owning both the country's telecoms firms.

Lord Ashcroft has strenuously denied Mr Barrow's claims that he held a monopoly over Belize's telecommunications industry, insisting that he sold Telemedia, a company he owned in the 1990s, seven or eight years ago.

Lord Ashcroft says the criticism is politically motivated. His spokesman, Alan Kilkenny, recently said the peer "has had no economic interest in telecoms in Belize for years". Mr Barrow stepped up the pressure last night by extending his attack to politics, describing Lord Ashcroft as a "relentless foe".

Asked whether he was concerned about Lord Ashcroft's influence in a Tory government, Mr Barrow said: "Yes, I am. I, of course, have to rely on the good sense of David Cameron, and I feel that he is sufficiently astute and wise... to be relied on, that I can expect that he would not allow one person to prejudice in any fundamental way the relations between our two countries. But I know that Lord Ashcroft clearly has to be influential – he is the deputy chairman of the party."

The Conservative leader has been criticised for not saying explicitly whether Lord Ashcroft pays tax in the UK, only that he is satisfied that there is not a problem.

In a sign of how Lord Ashcroft's largesse is paying political dividends, a poll for The Daily Telegraph yesterday showed the Tories streaking ahead in key marginals where the money has been directed.