Boris Johnson
London Mayor Boris Johnson pictured at Downing Street in May 2015. Getty Images

London Mayor Boris Johnson urged British government ministers to join the campaign to leave the European Union in a newspaper interview on Saturday, again defying Prime Minister and fellow Conservative David Cameron.

A political showman who is widely thought to be keen to succeed Cameron, Johnson said he wanted to change the minds of the majority of cabinet ministers who favor voting to remain in the EU in a June 23 referendum on the issue.

"People should look at the arguments. I have huge respect for what the Prime Minister is saying. But people I think should think about the arguments," Johnson told the Telegraph.

In a separate interview with the Times newspaper, Johnson dismissed the argument that a "leave" vote could spark last-ditch talks to achieve a better settlement for Britain inside the EU, resulting in a second referendum.

Asked whether there could be another referendum, Johnson told the paper: "No. Out is out."

Last week he had appeared to leave open the prospect of a second vote, arguing that the EU "only really listen to a population when it says No". That prompted Cameron to tell parliament on Monday that the idea of a second referendum was one "for the birds".

Opinion polls have differed about the likely outcome of the referendum, with the uncertainty sending sterling to a seven-year low against the dollar this week.

The latest poll, published on Friday by ORB for the Independent newspaper, showed support for the "out" campaign had risen to 52 percent from 48 percent from a month ago, while support to stay in the EU had fallen to 48 percent from 52 percent.

Cameron and Johnson have been friends since attending Oxford University together in the 1980s, but also rivals. In a thinly veiled jab at Johnson over the referendum, Cameron also told parliament on Monday: "I am not standing for re-election, I have no other agenda than what is best for our country."