Scotland "will have a referendum" if Downing Street does not reach an agreement with the Scottish Government over Brexit, the SNP's Westminster leader has said.

Angus Robertson warned Theresa May at Prime Minister's Questions on Wednesday in the House of Commons that she must honour her pledge to reach consensus across the UK before article 50 is triggered.

On a visit to Edinburgh in July, the Prime Minister said "won't be triggering article 50 until I think that we have a UK approach and objectives for negotiations".

Robertson asked the Prime Minister: "Only days remain until the PM is going to invoke article 50 on leaving the EU.

"Last July, she promised to secure a UK-wide approach - an agreement - between the devolved administrations - Scotland, Wales, Northern Ireland and the UK Government - before triggering article 50.

"So, when will the Prime Minister announce the details of the agreement?"

In reply to the Moray MP, May said she "will trigger article 50 by the end of March there will be an opportunity for further discussions with the devolved administrations over that period".

She also criticised Robertson's support for the EU over the UK.

The Prime Minister said: "He is comparing membership of an organisation [the European Union] that we have been a member of for more than 40 years with our country.

"We have been one country for more than 300 years. We have fought together, we have worked together, we have achieved together and constitutional game playing must not be allowed to break the deep bonds of our shared history and our future together."

May told MPs the devolved administrations will "have an opportunity for further discussions" before the end of March when she will formally notify the EU that the UK is leaving.

Robertson, however, warned May that if there is no agreement between Edinburgh and London then there will be an independence referendum.

He told the House of Commons: "The Prime Minister can wag her finger as much as she likes. Last year she made a promise - she promised an agreement.

"There is not an agreement. When will there be an agreement?

"Does she not understand that if she does not secure an agreement before triggering article 50, if she is not prepared to negotiate on behalf of the Scottish Government and secure membership of the single European market, people in Scotland will have a referendum and we will have our say?"