SNP MP Alison Thewliss is to force the first Commons vote on the 'rape clause' since the start of the controversial Tory-DUP pact keeping the Conservatives in power.

Under the controversial clause tax credits are limited to the first two children in every family – with an exception for women who can prove a third child was as a result of rape, or conceived during an abusive relationship.

Thewliss has written to DUPs MPs calling on them to break ranks with the Tories over the issue. She also challenged Ruth Davidson to instruct the 13 Scottish Tory MPs to vote against the policy.

Her call comes as Davidson is to deliver her keynote speech at the UK Tory conference in Manchester today.

Last night, Thewliss confirmed she would seek to force a vote on the policy after MPs return from recess next week. She claimed some DUP MPs had previously stated their opposition to the clause.

Speaking to the Sunday Herald, she said: “There is now no majority in the UK Parliament for this vile policy and I will be using all means I can to see it scrapped.

“I have previously secured the support of DUP MPs against the two-child policy and the rape clause, and I call on them today for them to renew their opposition and join cross party efforts to protect their constituents from these cruel changes.”

The vote would be the first on the clause since the DUP agreed to prop up the minority Tory government in exchange for £1 billion extra funding for Northern Ireland. The notional Commons majority of the Tory-DUP pact is six.

But in her letter to the 10 DUP MPs, Thewliss warned the that the policy is "dangerous and toxic" for women in Northern Ireland.

Thewliss said: “The impact of the two-child policy and the rape clause on Northern Ireland is dangerous and toxic.

The Glasgow Central MP also challenged Scottish Tory leader Davidson to abandon her support for the rape clause. Davidson has said she could be open to a review of how the clause works, despite refusing to condemn it.

“Ruth Davidson has suggested that the two child policy and the rape clause could be mitigated – but what does this offer women in Northern Ireland?

“There is no administration in Stormont, and the Tories in Westminster are still blithely disregarding the very real problems this policy has created.

"The chilling effect of the rape clause in Northern Ireland will discourage women from seeking support, which no-one, regardless of their party political persuasion, should want to see happen in a civilised society.

“The two-child policy and the rape clause must go across the UK – it is discriminatory, unworkable, and puts women in danger."

However, a spokesman from the UK Government's Department for Work and Pensions defended the clause.

The spokesman said: “This reform ensures people on benefits make the same choices as those supporting themselves solely through work. But we have always been clear this will be delivered in the most effective, compassionate way, with the right safeguards in place."

The DUP failed to respond to email and phone requests for a comment.