LONDON (Reuters) - Several Brexit supporters in Prime Minister Theresa May’s Conservative Party will seek to harden the government’s plan on leaving the European Union by changing her customs legislation in parliament next week.

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Jacob Rees-Mogg, a leading Brexit campaigner and the chairman of a group of eurosceptic lawmakers, said he was tabling amendments to the Taxation (Cross-border Trade) Bill, which will be debated and voted upon next week.

Illustrating the divided nature of May’s Conservative Party, rival pro-European lawmakers may also use the debate as a chance to further their push for even closer EU ties.

But as the government finally announces its negotiating stance, which is widely seen as a less radical departure than businesses and pro-EU campaigners had feared, the focus will be on Brexit supporters.

They have been angered by May’s proposal to keep strong trade ties with the EU after Britain leaves in March next year, a departure that marks the biggest foreign and trading policy shift in almost half a century.

The customs bill is a technical piece of legislation creating the legal powers for the government to put in place an as yet undetermined customs policy on imposing duties on goods entering the country. It will also have to be debated by parliament’s upper chamber before becoming law.

Rees-Mogg said his amendments would try to force the government to put into law its aim that Northern Ireland could not be treated separately, to stop what some Brexit campaigners see as attempts to divide the United Kingdom to ensure there is no return to a hard border with EU member Ireland.

The amendments would also: ensure “reciprocity of customs collection” to treat Britain and the EU as equals; put into law that Britain will not be part of the EU’s VAT regime; and require that any customs union would be created by primary legislation.

However, their main effect will be to reveal the extent of the unhappiness within May’s party.

Whilst the Conservative eurosceptic faction is large, without the support of the Labour Party - which looks unlikely - or a change of heart by the government, it will not have enough votes to force a change in policy.

A spokeswoman for May said she could not comment on the amendments without having seen them.