The battle within the cabinet to replace Theresa May as prime minister has left the UK an unreliable negotiating partner in the Brexit talks, unable to convince the EU that it will stick to any agreement it strikes, a former Irish prime minister has said.

Speaking to business leaders in Brussels, John Bruton accused the British government of being hopelessly divided, and offering only a vague and impractical vision of what may come once the UK leaves the bloc in 2019.

“If it got into detail, the disagreement between cabinet members is so deep that the Conservative party would split and the government would fall,” Bruton told the Institute of Directors. “The Labour opposition has a similar problem. UK party and public opinion has been polarised and is unready for compromise. The Conservative party is consumed with its leadership struggle and cannot be relied upon to make a deal that will stick.”

Bruton, who served as taoiseach from 1994 to 1997, and as the EU’s ambassador to the United States from 2004 to 2009, was speaking before a statement by May in the House of Commons in which she is to warn the 27 other member states that “the ball is in your court” after the concessions offered in her Florence speech, including €20bn (£17.8bn) to ensure no member states loses out in the two years directly after the UK leaves.

The European commission’s chief spokesman on Monday told reporters in Brussels that there was “clear sequencing” to the talks and “no solution” had been found to move the talks on from the opening withdrawal issues. “The ball is entirely in the UK’s court for the rest to happen,” the spokesman said.

The EU is refusing to address issues relating to a future trading relationship with the UK until sufficient progress is made on the issues of citizens’ rights, the Irish border and, perhaps most problematically, the UK’s financial settlement.

Asked about the absence of the Brexit secretary, David Davis, from the first day of the fifth round of negotiations starting on Monday, the spokesman added: “The European commission article 50 team is available 24/7. The timing of talks depends on the availability of our UK partners. We are always here and we are ready.”

The comments suggest there is little hope in Brussels that there will be progress in the negotiations this week. In Bruton’s address, the former Irish prime minister said he feared that the timescale allowed under article 50, under which the UK formally gave its intention to leave the EU, was simply too severe to allow for the negotiations to prosper. The EU, he said, was necessarily inflexible in its approach to the talks as it needed to determine its position by agreement in 27 European capitals.

This time-consuming process left a real risk of disaster as a transition period would also be difficult to negotiate, he said. The UK would have left the customs union and there could be need for protracted talks with both the EU and the WTO in order to organise a new temporary arrangement.

Bruton further warned that a deal on what EU regulations and directives would apply in the UK could additionally require ratification by all the member states, and their parliaments. He said the UK could find itself out of the EU in April 2019 before the transition deal had been ratified by all countries, meaning the deal would be inoperable..

“Negotiating and ratifying a transition deal could be almost as difficult as negotiating the final permanent deal,” he said.

Bruton said the UK should “engage itself seriously with the complexities of Brexit” and seek to extend the timeframe of article 50, something that would require unanimity by the 27 other EU member states.

While recognising this would be politically difficult both for the British government and the EU, Bruton said he feared it could take six years at least for an agreement between the two parties to come to fruition and such an extension may be the only way to avoid the UK leaving without a deal.

He said: “If it looks at all these complexities thoroughly, my sense is that it [the British government] may then conclude that, despite Boris Johnson’s anxiety to leave quickly, the time limits are already far too severe and that more time is needed … Of course this would be difficult. There are really no good options here. It is a question of deciding which option is the least bad one.

“The present tight timeframe minimises the opportunity for creative thought. Instead, it maximises the influence of blind bureaucratic and political forces. It increases the likelihood of miscalculation, and of the UK leaving the EU with no deal at all.”

Bruton told the Brussels branch of the IoD: “I hope more negotiating time can be agreed. If not, the tempo of the negotiation must be immediately, and dramatically, increased. Unfortunately, there is little sign that the current UK government, the originator of Brexit, sees this at all. Their mind is on other things.”