Sinn Féin leader Mary Lou McDonald has said the British government must introduce legislation for an Irish language act in Northern Ireland at Westminster.

She made the call after meeting Britain's Northern Secretary Brandon Lewis at Stormont tonight.

Sinn Féin has said the DUP must deliver on a commitment to deliver legislation before the next Assembly election, which is currently scheduled to take place next May.

The party says it will not agree to go back to power sharing if the promised legislation is not introduced.

The commitment was part of a cultural package in the New Decade New Approach agreement which laid the foundations for the restoration of the Stormont Assembly last January.

DUP leader Edwin Poots has said he is committed to implementing all aspects of the NDNA agreement, but that Irish language legislation will not be possible before next May.

The clock is ticking on the future of Stormont's powersharing Executive following the resignation of Arlene Foster as first minister.

Following the meeting with the Mr Lewis, Mary Lou McDonald said: "This evening we met with the British government and told them that they need to move the Irish language legislation through Westminster.

"A number of weeks ago the British government offered to legislate for Acht Gaeilge in this way.

"At that time we said that our preference was that Irish language legislation would be delivered through the Assembly and Executive as was agreed in New Decade New Approach.

"We have pursued that option vigorously over the last number of weeks.

"We have engaged intensively with the DUP and with party leader Edwin Poots. He has told us that they will not be delivering Acht in this mandate.

"This legislation was negotiated a year and a half ago and it is now incumbent on the British and Irish governments to act.

"This is the only way forward to finally resolve this issue."

An ongoing stand-off between Sinn Féin and the DUP over Irish language legislation could topple the institutions unless resolution is found in the coming days.

Mrs Foster's formal resignation today as joint head of the devolved Executive begins a seven-day timeframe within which the DUP must renominate its chosen successor, Lagan Valley MLA Paul Givan.

However, the joint nature of the office Mrs Foster shared with Sinn Féin's Stormont leader Michelle O'Neill, means Ms O'Neill must also be renominated to her role within those seven days.

If one of the parties fails to renominate within the time period, a properly functioning executive cannot be formed and the UK Government assumes a legal responsibility to call a snap Assembly election.

Earlier today, Ms O'Neill fired a warning shot to the new leadership of the DUP, stating that powersharing cannot be built on broken promises.

"You must deliver upon agreements that are made. I'm committed to do that. I hope others are also committed likewise to doing that," she said as Edwin Poots looked on.

Additional reporting: PA