Moore out as the reshuffle begins
A Government reshuffle is under way with Scottish Secretary Michael Moore joining the list of early casualties.
The Liberal Democrat's Cabinet post is being taken by the party's chief whip Alistair Carmichael - just a year before the crucial referendum on independence north of the border.
Cabinet Office minister Chloe Smith, deputy chief whip John Randall and transport minister Simon Burns have stepped down from the Government ahead of a shake-up of the Tory team.
There are also signs that Labour will carry out a reshuffle today.
Mr Moore is said to be the only Lib Dem Cabinet-level casualty planned, with Deputy Prime Minister Nick Clegg thought to want a more attacking approach in the run-up to the referendum.
The Prime Minister announced the appointment of Mr Carmichael on Twitter, also confirming that he will be making wider changes later. He is expected to promote women and northern MPs in what has been branded a "flat cap" reshuffle.
Ms Smith, who was only elected to the Commons in 2009 at the age of just 27, said in her resignation letter last night that she wanted the chance to "develop other ways of giving public service, both inside and outside Parliament".
As economic secretary to the Treasury she once suffered a humiliating verbal mauling at the hands of Newsnight presenter Jeremy Paxman over a fuel duty freeze.
The interview led to Chancellor George Osborne being branded an ''arrogant coward'' after sending her into the TV studios to defend his decision to delay the 3p hike in fuel duty.
Mr Cameron applauded her "positive impact in the departments you have served" and said he looked forward to receiving her recommendations on encouraging young people to vote.
"As you know from our previous conversations, this is a topic I take seriously and believe that there is scope for action," he told her.
Mr Randall, 58, has been the Conservative MP for Uxbridge and South Ruislip since a 1997 by-election and told the Prime Minister at the end of last year that he wished to step down.
He said it had been "a great privilege and honour" to serve for 13 years in the Whips' Office in opposition and Government.
The Prime Minister said he "could not have wished for a more loyal, discreet, patient, trustworthy and committed colleague" and that he "had rather hoped this day would never come".
Don Foster, who was a junior minister in the Department for Communities and Local Government, has replaced Mr Carmichael as Lib Dem chief whip.
Conservative MP Richard Benyon announced on Twitter he is no longer Fisheries Minister.
He wrote: " On back benches! 3 and half really fun years with much achieved. Really appreciate time working with outstanding Ministers and Officials."
Tory Greg Hands has been promoted from assistant whip to Deputy Chief Whip, No 10 confirmed.