Andrea Leadsom, Theresa May's new Leader of the House of Commons, made the announcement on Thursday morning as talks continued between Government ministers and the DUP.
Sky's David Blevins says sources have confirmed there is still no agreement between the Conservatives and the DUP, despite the new date for the Queen's Speech, which was originally meant to be on 19 June.
In a statement, Mrs Leadsom said: "The Government has agreed with Buckingham Palace that the State Opening of Parliament will take place on 21 June 2017."
The delay to the Queen's Speech comes after the Government lost its majority at last week's General Election, leaving Mrs May trying to secure the DUP's support to prop up her minority government.
DUP MPs Nigel Dodds and Sir Jeffrey Donaldson are locked in negotiations with First Secretary of State Damian Green and Gavin Williamson, the Chief Whip.
The leader of the DUP, Arlene Foster, has flown home.
The State Opening of Parliament was due to happen on the same day Brexit negotiations were meant to begin.
It is not yet clear whether official talks about the UK's withdrawal from the EU will go ahead on 19 June still, but Brexit Secretary David Davis has said they will start "next week".
Mrs May was due to hold talks on Thursday with other Northern Ireland political parties amid fears the Government's relationship with the DUP could undermine the peace process.
On Wednesday, former prime minister Sir John Major, one of the architects of the peace process, said the Government will compromise its impartiality if it enters a confidence and supply deal with the largest Northern Irish party.
The 1998 Good Friday Agreement, also referred to as the Belfast Agreement, requires the UK and Irish governments to show "rigorous impartiality" as they deal with the different political groups in Northern Ireland.
The power-sharing agreement between the DUP and Sinn Fein broke down late last year.
Northern Ireland Secretary of State James Brokenshire said an agreement to restore the power-sharing government in Stormont must be reached by the 29 June deadline or direct rule will be restored.
