By David Milliken
LONDON (Reuters) - The official measure of Britain's trillion-pound government debt will rise by more than 100 billion pounds in September, as part of a major revamp of the politically sensitive numbers by the country's statistics agency.
Some of the changes are due to new European Union statistical rules, which mean the not-for-profit body which operates Britain's rail network will be classified as a public rather than a private corporation.
But the bulk are an attempt to clean up how Britain's public accounts have been presented since the financial crisis, which saw the purchase of big stakes in Royal Bank of Scotland and Lloyds Banking Group, as well as of over 375 billion pounds of government bonds by the Bank of England.
The changes are almost wholly in line with proposals the Office for National Statistics made in December, when one of its officials described the existing statistics as "no longer fit for guidance, no longer fit for purpose".
Given the constituent parts of the new debt total are already available, there should be no surprises for financial markets.
Indeed, many private-sector analysts and the government's budget watchdog already use a modified version of the ONS's headline measure of the public finances which additionally strips out some payments linked to Bank bond purchases and Royal Mail Group's historic pension assets and liabilities.
Credit ratings agencies prefer another measure, general government gross debt, which is more internationally comparable.
However, the ONS proposals go further - and in the run-up to a national election in May 2015, raise questions about how easy the new numbers will be to compare with past government plans to reduce Britain's borrowing.
Data in the old format will cease to be published after April 2015. The new-style figures will be available from June, and will be used as the main headline measures from September.
The ONS plans to replace its current headline measure of Britain's total outstanding borrowing - public sector net debt excluding financial sector interventions - with a new measure, public sector net debt excluding banking groups.
The ONS said that the change - if implemented already - would add 113 billion pounds to Britain's public sector net debt for the 2012/13 fiscal year, when it already stood at 1.185 trillion pounds or 74 percent of gross domestic product.
Some 30 billion pounds of that increase is due to the change in classification of Network Rail, required by the EU.
Of the remainder, 25 billion pounds comes from a changed treatment of the BoE's bond purchases and 58 billion pounds from no longer counting the government's stakes in RBS and Lloyds and payments into a bank bailout fund as liquid assets.
However, the changes will slightly reduce annual borrowing on the measure favoured by the government's budget watchdog, public sector borrowing excluding the payments related to the Royal Mail pension plan and the Bank's bond purchases.
This totalled 114.9 billion pounds in 2012/13 - 7.3 percent of GDP - but would have been 3.4 billion pounds lower at 111.5 billion pounds if the new measures had been used.
(Editing by Toby Chopra)