Theresa May was accused last night by a former cabinet colleague of planning the “biggest giveaway of sovereignty in modern times”, as she faced a potentially devastating pincer movement from Tory remainers and leavers condemning her Brexit plans.

The day after Jo Johnson, the pro-remain brother of former foreign secretary Boris Johnson, resigned from the government and called for a second referendum on Brexit, former education secretary Justine Greening launched an attack on the prime minister, saying her plans would leave the country in the “worst of all worlds”.

Piling yet more pressure on May, Greening – who resigned from the cabinet in January – backed the former transport minister’s call for another public vote and said MPs should reject the prime minister’s deal. Greening told the Observer: “The parliamentary deadlock has been clear for some time. It’s crucial now for parliament to vote down this plan, because it is the biggest giveaway of sovereignty in modern times.

“Instead, the government and parliament must recognise we should give people a final say on Brexit. Only they can break the deadlock and choose from the practical options for Britain’s future now on the table.”

Greening added: “Like many of us, Jo Johnson is a pragmatist on Britain’s relationship with the EU. But Conservative MPs can increasingly see that this sovereignty giveaway from No 10 leaves our country with less say over rules that govern our lives … That is not in the national interest, it’s the worst of all worlds and it resolves nothing.”

Meanwhile Sky News reports that Theresa May is facing a battle to save her Brexit plan amid claims the European Union has rejected a key proposal. Talks with Brussels have reportedly broken down over the PM’s solution to the Irish backstop.

The current proposal would see the whole of the UK remaining in a temporary customs arrangement to avoid a hard border between Northern Ireland and the Republic.

To allay fears this would leave the UK trapped indefinitely in a customs union – Mrs May had drawn up a mechanism which would allow the UK to leave the backstop.

Using the legal expertise of Attorney General Geoffrey Cox, it proposed using an independent arbitration panel.

But the EU has reportedly dismissed this plan, dubbed the “Cox compromise”, insisting any arbitration must come from the European Court of Justice.

This would be a huge blow for the PM’s hopes of reaching a deal with Brussels by the end of the month.