The Church was formed when English Presbyterians formally united with English and Welsh Congregationalists to create the first union of separate denominations since the Reformation.
The URC's creation was the result of The United Reformed Church Bill, which was introduced into the House of Lords in November 1971 and eventually passed on 22 June 1972. The first meeting of the United Reformed Church Assembly took place on 5 October 1972.
In 1981, the URC united with the Association of Churches of Christ and then in 2000, joined together with the Congregational Union of Scotland.
Today there are around 1,600 URC congregations with around a hundred thousand members in Britain today.
The Rev Roberta Rominger, general secretary of the URC, has written a list of 15 things to celebrate about the URC. Her list includes the Church's Children's Assembly and its long history of ordaining women.
She said: "I look back over the past four decades of our history and I see great things and not-so-good things, but mostly I see a feisty church, made up of feisty Christians, called to be God's people, transformed by the Gospel of Christ and making a difference in the world.
"I am proud of this denomination. There is much to be proud of: our wealth of fantastic hymn writers; our early commitment to the ordination of women; our model of concilliar not hierarchical leadership; our church related community workers, combining professional community work with gospel values and the fact that we consistently punch above our weight, campaigning on a whole range of social justice issues including the use of drones, the financial transaction tax and issues of poverty and inequality. Bravo the United Reformed Church."
A hymn has been written by Alan Gaunt to celebrate the URC's 40th birthday and the Rev Richard Bittleston has penned a special prayer:
A prayer for the URCs 40th Anniversary
Flowing river of love, on which we are carried,
taking us to places of renewal and refreshment;
we pray for the courage to trust the direction in which you take us.
For all the sights we have seen, all the stories we can tell;
we smile at your great skill and give thanks.
As we strain to see what lies around the corner,
may we also know when to set the oars of our boat at rest.
And when we have drifted enough to understand your direction;
and more fully appreciated the view of both shore and sky line;
may our energy meet with the current and carry us on.
In your love, in your way, in your creation.
Amen