The political debate has stepped up a notch in recent weeks, as Boris Johnson and Ken Livingstone once again square up ahead of the Mayoral election next year.

Full Fact today received confirmation that the UK Statistics Authority would be raising concerns with the Home Secretary, the National Statistician and the Minister for the Cabinet Office with regard to Mayor Johnson’s use of unpublished statistics at press briefings after we highlighted the issue.

This weekend saw his opponent make some transport claims of his own, arguing on the BBC’s Politics Show that “we already have the highest fares in the world, why on earth should we increase fares greater than inflation for 20 years?”

Factchecking a claim like this is notoriously difficult, as making international price and cost comparisons needs to take into account issues such as exchange rates and relative purchasing power between countries. This means that results tend to be qualified.

UBS does however research and publish a price and earnings comparison report, seeking to make international comparisons using a New York price benchmark. In their most recent full report in 2009, London was ranked as the most expensive city in the world for transport. London fares averaged around double those charged in Western Europe and were 3.4 times larger than the global average.

Interestingly, the report puts London at number 21 in the rankings of the world's most expensive cities. This was mainly down to the fall in the value of the pound; with London having ranked number two in the 2006 lists. This case is a good indicator of the limitations and time sensitivity of international price and cost comparisons.

However London hasn’t always been the most expensive city for transport. Indeed one need only look to the 2006 UBS report on price and earnings to see the last time that London did not top that particular league table.

But whilst this might suggest that Londoners have seen rises spike under Boris Johnson’s new administration, there is an important caveat that needs to be noted.

In 2007, when Ken Livingstone was still Mayor, the Guardian reported that London's transport network was set for “three years of 10 per cent price rises.” In commenting on the news, Mr Livingstone argued that the rises were necessary “in order to pay for massive investment in our public transport system, including new rail links and better services.”

Whilst Mr Livingstone is of course free to argue against any further price rises, it may not be entirely fair to pin the current cost of transport on the present Mayor.