Mr Howard was reportedly told he would not meet Mr Bush

The Sun reported senior aide Karl Rove told Mr Howard in February: "You can forget about meeting the president full stop. Don't bother coming."

The officials were reportedly furious at the Conservative leader's call for Tony Blair to resign over the Iraq war.

Mr Howard said he would carry on doing his job as he saw fit and accused US aides of trying to protect Mr Blair.

If some people in the White House... think I am too tough on Mr Blair or too critical of him, they are entitled to their opinion Michael Howard

"A Conservative government would work very closely with President Bush or President Kerry, but my job as leader of the opposition is to say things as I see them in the interests of our country and to hold our government to account," he said in a statement released on Saturday.

"If some people in the White House, in their desire to protect Mr Blair, think I am too tough on Mr Blair or too critical of him, they are entitled to their opinion.

"But I shall continue to do my job as I see fit."

The row runs contrary to a tradition of political alliance between the Republicans and the Tories, epitomised by the friendship between Margaret Thatcher and Ronald Reagan in the 1980s.