Deputy Prime Minister Nick Clegg is due to mount a vigorous defence of the coalition's welfare reforms, insisting the Government had an "absolute duty" to ensure the system was fair to all.

While acknowledging the changes had at times been "painful and controversial", he will use a keynote speech to argue that the Liberal Democrats had ensured they were firmly anchored in the political centre ground.

He will say that without reform, there was a risk of a "total collapse" in public support for the whole principle of welfare.

His speech - on the eve of the fifth anniversary of his election as party leader - comes at a difficult moment for the Lib Dems with a series of weekend opinion polls showing them slumping to fourth place behind the UK Independence Party, with their support down to just 8% or 9%.

In his address to the Centre Forum think tank, Mr Clegg will acknowledge that governing in difficult times had meant the party had acquired a "harder edge", but he will say the alternative was "a retreat to the comfort and relative irrelevance of opposition".

With the welfare system they inherited from the former Labour government both badly designed and financially unaffordable, he will say the coalition had no choice but to carry through major changes.

"When two thirds of people think the benefits system is too generous and discourages work then it has to be changed, or we risk a total collapse in public support for welfare existing at all," he will say, according to advance extracts of his speech.

"We need welfare protection for people who fall on hard times. Of course. But you cannot ask low income working people to pay through their taxes for people who aren't in work to live more comfortably than they do."

While the Lib Dems had ensured that protections for the most vulnerable were built into the system, Mr Clegg will emphasise that reform was not forced on them by Conservatives.

"It was in our manifesto and on our agenda right from the start. The Liberal Democrats are now the party of welfare reform - sensible, centre ground welfare reform," he will say.