Two rebel Conservative MEPs who voted to block moves towards trade talks between the UK and European Union have been stripped of the party whip. The South West England MEP Julie Girling and South East England MEP Richard Ashworth were suspended from the party after supporting a resolution in Strasbourg declaring that “sufficient progress” had not been made in the Brexit talks to move on to discussions on the future relationship between the UK and EU.

The decision to remove the whip was backed by Downing Street in a sign that Theresa May is preparing to get tough with her party over Brexit as the talks move into a crucial phase. The prime minister’s attempt to assert her authority comes after a mishap-strewn conference speech and the foreign secretary, Boris Johnson, setting out his own “red lines” for the Brexit talks.

The whip was removed from the two senior MEPs after consultation between the party’s European parliament chief whip, Dan Dalton, and Downing Street. Ashworth, a former Tory group leader in Strasbourg, and Girling, a former chief whip, backed the non-binding motion, which was passed by 557 votes to 92, with 29 abstentions.

In his letter to the pair, Dalton said: “The Brexit negotiations are the most important negotiations our country faces and reaching a new partnership with the European Union is in the interests of both the UK and the EU.

“The resolution by the European parliament sought to delay progress in the negotiations between the UK and the EU by holding back talks on the future relationship. It also proposed that one part of the UK, Northern Ireland, could remain in the single market and customs union, while the rest of the UK departs – which is not acceptable.

“Given the seriousness of this issue, and your failure to discuss your intention to vote against the agreed position of the Conservative delegation in advance, I am therefore writing to inform you that I am suspending the Conservative whip from you until further notice.”

A government source said Downing Street had backed the move because the rebel MEPs had behaved “totally irresponsibly”. The source said: “Regardless of how you voted in the referendum, it is surely in everyone’s interests – both in Britain and in Europe – that talks can progress on trade and our future relationship.”

