The snap agreement by the four leading candidates to be London mayor to publish how much they pay in tax points to the increasing Americanisation of British politics.

Their agreement came during a mayoral debate on BBC Newsnight on Wednesday, itself something of an American procedure, is likely to increase pressure on members of the Cabinet to do the same.

A brief row over how much senior politicians earned and whether they would benefit personally from the cut in the 50p rate of income tax following the Budget shows the appetite for the release of such information at a time of squeezed spending and frozen income levels.

The practice of releasing tax returns is common in US elections, and the precedent for a presidential candidate releasing their details was set by Republican George Romney in 1967.

During his campaign for the presidency that year, George Romney made the groundbreaking decision to publish 12 years worth of tax returns. "One year could be a fluke," he said. "Perhaps done for show"

George was of course, the father of Mitt, the current front runner to take on Barack Obama in this years presidential election.

Mitt Romney, the millionaire businessman and former governor of Massachusetts himself ran into trouble after initially resisting demands from rival Republicans to release his tax information.

Having relented it was shown that while he earned $21.7m in 2010, he paid a tax rate of just 13.9 percent, a lower rate than that of a person earning $50,000, due to the way he organised his bank accounts.

In an interesting parallel with the London mayoral candidates' agreement to publish their tax details during a debate, it was a botched answer during a televised debate that heaped pressure on Romney in January.

Asked whether he would release information about his tax payments, Romney merely responded "maybe". A response that drew boos from the Republican crowd.

He released one year's worth of information four days later.