Downing Street has begun an inquiry into a string of government leaks to Sunday newspapers.

The prime minister is reported to have personally ordered the probe into how the Sunday Times in particular has come upon a series of key Cabinet documents.

However, embarrassingly for the government, news of the inquiry itself was leaked to the same paper.

It reported on Sunday that Cabinet Office permanent secretary Sir David Omand has been put in charge of the investigation, reporting directly to Cabinet secretary Sir Andrew Turnbull.

Ministers want to get the to the bottom of why the Rupert Murdoch-owned broadsheet has been able to break exclusive stories on subjects including the honours system, identity cards, alcohol-related crime and concern over "heavy-handed" US tactics in Iraq.

In a presentation to the Cabinet Sir David said 26 documents have found their way into the hands of political editor David Cracknell in 15 months.

Sir Andrew said "this cannot be allowed to continue". "I therefore think that we have reached the point where we need to take stronger action to counter it," the head of the civil service wrote.

"There are several leak inquiries in train and I have decided to put more resources into the main series of leaks - to the Sunday Times - and to widen it to cover more departments."