LONDON (Reuters) - Pro-Brexit members of parliament heaped pressure on British Prime Minister Theresa May on Wednesday over her future customs plans with the European Union, calling on her to drop what some say is her preferred proposal.

Before May met her so-called Brexit war cabinet, a group of MPs criticised her proposal for a customs partnership, saying the arrangement that would see Britain essentially act as the EU’s tariff collector was unworkable.

Her spokesman said the government’s ideas were evolving.

May’s decision to leave the EU’s customs union, which sets tariffs for goods imported into the bloc, has become one of the main flashpoints in the Brexit debate in Britain, pitting companies and pro-EU campaigners against a vocal group of hardline eurosceptic MPs.

With the added pressure of trying to prevent the return of a “hard” border in Ireland and find something Brussels might agree to, May has delayed putting any firm plans for future customs arrangements on the table, hoping to plot a route that could at least please more than one side.

Her challenge was writ large when members of the European Research Group, a group of MPs in May’s Conservative Party, said they had presented their argument to the government that a customs partnership would not work.

“It is more of a statement of our position, with supporting arguments,” a member of the ERG said, denying it was an ultimatum to the prime minister or an attempt to undermine her.

Some pro-Brexit MPs fear the customs partnership is little more than an attempt to stay in the customs union -- something they say would not deliver the kind of divorce the government has promised to pursue.

May is not only under pressure at home.

She also faces increasingly urgent demands from Brussels to come up with a customs plan to avoid a return to a hard border between British-ruled Northern Ireland and the Irish Republic, which some fear could reignite sectarian violence.

In Dublin, Irish Prime Minister Leo Varadkar said there was a risk that Britain and the EU would fail to reach a withdrawal deal by an October deadline unless “real and meaningful” progress was made by a summit in June of the bloc’s leaders.

EVOLVING

After losing her party’s majority at an ill-judged election last year, May has put off committing to a single plan, offering Brussels two options -- the customs partnership or a technology-based streamlined customs arrangement, both of which EU negotiators have dismissed.

Britain's Prime Minister Theresa May walks out of 10 Downing Street in London, Britain, May 2, 2018. REUTERS/Hannah McKay

In parliament, May said there were a number of ways to solve the customs issue, in what seemed to suggest that her government could be looking beyond the two proposals already made.

Asked whether there were other options, her spokesman said: “Work has been ongoing on two options, that work has been proceeding. Ideas obviously are evolving as we go along and the prime minister said there are a number of ways to proceed.”

The easiest way to solve the problem, May’s critics say, is to stay in the customs union or negotiate a new one along the lines proposed by the main opposition Labour Party.

Her spokesman said earlier this week the government would “move forward with a single option”, but the question is when?

On Wednesday, her minister for the cabinet office, David Lidington, said it would most probably take a few weeks to decide on a final position, playing down any expectations of a quick decision at Wednesday’s meeting of the Brexit committee.

Brexit campaigners are hoping that the appointment of free-market advocate Sajid Javid as Home Secretary this week could shift the balance in their favour on the sub-committee.

But by leaving the question open May has been vulnerable to attempts both in the upper and lower houses of parliament to try to force the customs union back onto the agenda.

Her government was defeated in the House of Lords earlier this week, and has postponed votes in the House of Commons after several MPs in her party said they would support attempts to draw a commitment to stay in the customs union.