Nick Clegg has insisted that he is not "acting prime minister", despite taking over much of David Cameron's work while he is on holiday.
The deputy PM said he was just "holding the fort" and Mr Cameron was "still in charge".
Mr Clegg, speaking at a public meeting in London, also predicted the Tory-Lib Dem government would remain in power for a full five years.
Mr Cameron is on holiday in Cornwall for the next two weeks.
The deputy prime minister, leader of the Lib Dems, has said he will argue for key elements of his party's agenda during this time.
He is undertaking a series of public meetings around the UK.
'Spirit of partnership'
Speaking at an event organised by MSN in central London, Mr Clegg said: "I'm not acting prime minister... The prime minister is now taking a well-deserved break. It's a quite straightforward situation.
"The prime minister is the prime minister. He continues to stay in charge...
"Obviously I'm holding the fort for a couple of weeks and doing it in the spirit of partnership which is at the heart of the coalition government."
Asked if he could have foreseen being in this position before the general election, he replied: "No."
Mr Clegg insisted that the, which has been in power for almost 100 days, would last for a full five-year term.
He said: "I think we need five years to sort things out... to take a lot of difficult decisions so that we can move forward to rebuild the economy."
Mr Clegg was quizzed about the government's planned spending cuts, likely to be the biggest in the UK for decades, saying that they were necessary to cut the budget deficit, but adding: "I think there's light at the end of the tunnel...
"We hope that, when the five years are up, that people see we have taken the right decisions.
"I think a lot of people felt that a coalition government would be, by definition, some sort of insipid mush...
"Actually, what we are finding now, after 100 days, is that we are being accused of doing completely the opposite: being too radical, too reforming."
Since sealing the coalition deal in May, the Liberal Democrats have seen their poll ratings decline substantially, while the Conservatives and Labour have picked up support.
While acting as caretaker during Mr Cameron's absence, Mr Clegg's task is also to ensure the junior partner of the coalition can punch above its weight, said BBC political correspondent Iain Watson.
Later this week he will address criticism within his own ranks that the coalition is balanced too far towards Conservative interests and too focused on public spending cuts.
On Sunday, Liberal Democrat deputy leader Simon Hughes said his party was determined to fight the Tories for seats in the next general election.
He told the BBC the coalition was a business arrangement, not a marriage.
"You fight the election on your own and we will do it because we want to win more seats," he said.
"We want to have more influence and we want to be in government ideally on our own because evidently if we have a majority we can implement more of our policies and not only a proportion."
Over the next two weeks Mr Clegg will also travel extensively in England outlining plans for greater opportunities to the least well-off and to encourage job creation outside of the South East.