Ministers had warned that defeat would trigger a constitution crisis, yet the upper house voted for the changes to be delayed until a three-year package of transitional financial help for those affected has been agreed upon.

A second motion from Baroness Meacher - calling for the cuts to be delayed until independent analysis of their effects was carried out - was also supported.

She told Sky News: "I think it's a very powerful message that now goes to the Government."

But Tory MP Michael Ellis said: "We can't have a situation where the unelected peers overrule the elected House of Commons."

Chancellor George Osborne said: "Unelected Labour and Lib Dem lords have voted down a financial matter passed by the elected House of Commons. This raises constitutional issues."

However he said that transitional arrangements to help people hit by tax credit cuts will be set out in the Autumn Statement.

The tax credit reductions were due to come into effect in April, but will now be put on ice.

Earlier a "fatal" motion to scrap the cuts altogether was defeated.

Shadow chancellor John McDonnell said: "These cuts would hit working people the hardest. It's just unacceptable, not just to the House of Lords but also to MPs on all sides in the House of Commons."

He added that there would be "outrage" if the Government tried to add scores of new Tory peers to the Lords in future to ensure legislation could be passed more easily.

Tax credits were introduced by Labour to help low-paid families.

Opponents of the Government's changes say millions of existing recipients - many of whom work but are on low incomes - could be left up to £1,300 a year worse off.